I think it's just a man thing. I'm sure he still loves you and finds you attractive.
Maybe he not ready to be a father so he not in the mood or he could just have a lot on his mind guys are so wired just ask him what up
I am 21 as well and my boyfriend is 27. He did the same thing to me when I started showing. This is our third baby so he isn't new to my body changing. I felt the same way and I almost have to rape him to get anything done. Try telling him how you are feeling. Guys libidos go up and down just like ours do so hang in there it will pass