Sick chinchilla

My 12-year-old chinchilla, Misty has had quite a few signs of sickness over the past few weeks. I am posting this to see if anyone has heard of any of the symptoms she has had or has/had a chinchilla with similar symptoms and some advice they can offer. My chinchilla has the following symptoms:
1. Pawing at mouth (she has had this issue twice before, we took her to the vet both times, we're not sure of the cause, they gave her a shot and some medicine which seemed to work as a temporary fix
2. Messiness in food eating (when she eats, a majority of the food seems to fall out of her mouth in crumbs, and she also seems to push some of it out, linked to pawing at mouth, is recurring)
3. Weight loss (we have been keeping track of Misty's weight since March of this year and in the past few weeks her weight, which had been around the 15.5-16 oz mark, has been lowering alarmingly fast down to the 15 oz mark. She used to be around 16.7 - 16.0 oz mark but after the last time she went to the vet she hasn't really recovered weight-wise. She feels alarmingly thin when you feel around for her ribs as well.
4. Weird black stuff on teeth? (I'm not sure if this was there before. I have found a way to easily let me see Misty's teeth, so I might have just not spotted it before, it could just be a thing she has as a mark. On her bottom front teeth, in the middle, there is black stuff on it. It doesn't seem to be food or anything that would be removable. I do know she had a cagemate before we adopted her that had some sort of tooth problem that ultimately killed her, but I cannot remember the details and although she got tested for the tooth problem at the time it came up negative)
5. Heart Murmur (The first time we took Misty to the vet, we learned she has a heart murmur, but we haven't gotten to treating it just yet, although we probably will eventually. We do not know if this is causing her problems, and we don't have any real idea why she has a heart murmur, but she does)
And I believe that is it. So sorry for making such a long post, I'm just really concerned about her and I want to see if anyone knows anything based on their experiences. I would also like to say we will be taking Misty to the vet very soon.

It sounds like a tooth or mouth problem, the pawing at the mouth, messy eating, and weight loss are are signs of something wrong going on in her mouth. Blackness on the teeth make me wonder if she could have possibly chewed on an electric cord, that would leave black burn marks on her teeth and would hurt causing the pawing at the mouth, dropping food, and because it hurts to eat, weight loss. Another thing that can cause the teeth to turn black is if there was trauma to the tooth or teeth causing them to die.

Just for future weigh ins, it's best to weigh them in grams, they weigh so little that grams give you a more precise measurement. Going from 16.7oz to 15oz is almost a 50g loss (weigh fluctuations of about 10-20g is normal, so the half an ounce +/- is within normal), anything more then that, especially if the weight is only going down not back up, is a sign something is wrong. Chins are very good at hiding health issues so weigh loss is often the first, and sometimes the only sign something is wrong, by the time they actually act sick or in pain they are often doing very poorly because it means they can't physically hide it anymore.

Do you mean Misty was checked by the vet for tooth issues or the cage mate that died was? If it was Misty what do you mean by "tested for tooth problems", do you mean they took x-rays or just used a camera scope? If she hasn't been checked for tooth problems I would definitely get her checked out. If they checked her teeth and just did a camera scope and didn't see any issues, then you need to get x-rays taken to fully see if the roots are a problem. In chins not only do the crowns of the teeth (the part of the teeth you see above the gum line) grow (front and back teeth) for the chin's whole life but the roots are open ended too so they can also grow up into the skull and down into the jaw. If they did actually check the teeth then I'm guessing they likely just gave her pain meds, which would make her feel better for a little while until they wear off, but that doesn't fix the problem.

Heart murmurs are common in chins, she likely has had it for awhile, and lots of chins live perfectly normal lives with heart murmurs. It's just something to keep in mind if she every has to go under anesthesia, and also make sure she doesn't over do it during playtime, she may also not be able to have a wheel depending on how bad it is. It's also a good idea to have them check her heart yearly to make sure the murmur hasn't gotten worse, unless the vet has already said it needs treatment, but it wouldn't cause problems with the mouth.

Misty was checked for the same tooth problems as her cagemate, and they did take x-rays. My information on this is limited since she is a rescue and it has been a few years since we've adopted her, so I'm not sure on what the tooth problems were.

Thanks for the response by the way, this is very informational and makes me feel better

Definitely sounds like something going on with her teeth and/or mouth. Just because her teeth checked out a few years ago doesn't mean she hasn't developed tooth problems since then. I'd definitely make her an appointment with a vet to have her teeth checked again. Her pawing at her mouth, messy eating, and weight loss all point to something going on with her mouth. It sounds like it's painful for her to eat.

I hope the vet is able to figure out what's going on and get her fixed up and eating well again!

I have to agree with the previous responses, that this seems like a tooth problem. Unfortunately an xray often does not show up the problem and a (very expensive) MRI may be needed to properly identify that a tooth has gone rogue and is impacting the tear duct / growing out through the bottom of the jaw / rubbing the side of the cheek / impacting on the nerve that runs at the lower jaw etc.

Anti-inflamitory painkillers can temporarily remove the symptomatic behaviour whilst leaving the underlying problem untouched.
In other cases it can be one or more teeth are growing sideways an impacting onthe side of the cheek. This can be treated succesfully by having the offending teeth ground every 2-3 months and the chin will return to enjoying life.

Best of luck for a quick diagnosis and a return to health for your furbaby.