Yes, I noticed her belly is rather hairless. For pregnancy, the hairs around the nipple will thin out, but her entire belly shouldn't happen. I have seen it on pregnant cats before, but those just had babies and where pregnant again. I assume this cat didn't have kittens yet.
If not, this might be a medical condition. I would suggest visiting your vet if the hair doesn't start to return anytime soon or continue to fall out on her belly or other places.