My cat Pearl who is a Lynx Point Siamese mix will also tend to lose her fur on her lower part of her tummy. When she was in last year for her annual exam, I pointed this out to my vet. She said she's seen much worse so she wasn't too concerned about it. I notice that it comes and goes. I haven't seen her overgrooming but I'm sure that this is what she is doing. She doesn't scratch and she doesn't have red spots like an allergy or anything like that. She is stressed out because she doesn't get along well with most of my other cats so she basically lives in my bedroom/bathroom area. Lately she's even been fighting with Ziggy and Sky which are the only two cats that she even tolerates.

I would just watch your cat. It may also be due overgrooming caused by stress. Has anything changed for her lately?