They can get fleas on any area that doesn't have thick fur, like feet, eats, or if the chin's fur in bad shape with lots of missing fur causing bald spots. Otherwise fleas don't live long on chins because the fur is too thick, they suffocate, solving the issue on it's own. If she isn't acting back to normal, or she is in poor shape I would take her to the vet, they would be able to clean the fleas out of her coat or figure out what is wrong. Flea meds for cats would likely kill the chin, the drops are basically poison that absorbs into the skin, so if the flea bites it dies, but even if it's safe for use on a newborn kitten (which I doubt) doesn't mean it's safe for a rodent, though you can call the vet and ask.