You are packed full of baby right now and the very thought of try to put something else in that small cramped space is a chore. Don't feel bad saying no, or just find other ways that aren't uncomfortable, it's just very important to communicate with your partner so you both know what you want. Sex in the third trimester is a big undertaking don't worry this too will pass.
There's a lot going on down there so it's a very sinsitive area right now that sounds like a normal issue.