Is it a longhair or short? It could be losing puppy coat and longhaired chis would probably look as if they're balding around that time. If you're going to the vet for more puppy shots soon, I'd ask the vet - they may do a skin test to ensure it's not a skin infection. You may want to call the vet anyway- better safe than sorry.
i've seen it a lot in short hair chi's. when chiwi was blowing her puppy coat she didn't have hair loss in that area at all (she is long hair) next vet appointment just point it out and get a skin scraping so you can have it checked for mites. just to be sure.