I don't disagree with either of Mish's suggestions. I disagreed with the indication in both previous posts that it was not possible to have another pregnancy. While I said it was possible, I don't think it is probable, and never stated it as such, simply that it could happen.
Personally I don't think it's a retained kit. Usually with a retained kit IME the mom never bounces back after delivery and stays in a semi lethargic state, often eating and drinking little and in turn her born kits suffer due to lack of milk production, they will look ill, she will look ill. By a week and a half the mother was be very septic by now as there would be a rotting kit inside of her. This is my personal opinion, I'm not a vet. I personally dis-reagrded this and asked about the poo which can help check for signs of bloat, to which the OP never answered yet.
Similar to what Mish said, I don't necessarily agree that the belly is an issue either. Usually for a few days the belly is flaccid as Mish says but it firms back up quickly as the uterus shrinks. A chin's belly SHOULD be firm, not soggy or over extended ( too firm ). Usually in the time after birthing a mother eats more to care for her kits and to replenish the nutrients her body used to grow them and deliver them, which means she puts out more waste, and has more fecal material in her intestinal tract.
I personally think if anything it would be poss. bloat, again my opinion. I would check the poo, assuming there is plenty of intake and output I would use a probiotic to encourage good digestion, if there are a lot of holes in the poo I may give some simethicone to relieve that aspect while the probiotics get a chance to work.
ETA: The OP stated while I was typing my novel, that they felt the belly during pregnancy, did you feel it before she was? I have females who ALWAYS have a firm belly like a pregnant chin, whether they are pregnant or not. I have others who are softer, pregnant or not. Personally, and as Mish indicated, it is probably NOT a second pregnancy. There is more to pregnancy than a firm belly, firstly embryos... but other than that, other things as well. So how's the poo?