You are right, it does sound like all her pants are preggers. One is descending a slippery slope when one tries to correct the grammar of others. (That doesn't sound right, either!) ;)
I think you should have said: being 6-7 months pregnant, all of her pants are tight fitting, I bet OR I bet that, because she is 6-7 months pregnant, all of her pants are tight fitting OR she's fat