No Sex Drive (Possible TMI)

So, my boyfriend and I have been together for 3 and a half years. The first few months we would have sex frequently, but it would start to drop off rapidly due to me just not being interested. I'm pregnant for the second time now (26 weeks) and just have no desire to even touch him - it's like a chore. It's nothing personal, I'd love to be more intimate with him again... I just don't know why I can't bring myself to enjoy him sexually. I still find him attractive, we spend a lot of time together and not enough at times, it just doesn't matter. He will try to initiate it, but I'm just not very motivated. He's been very understanding, but it bothers him, I just want to go back to "normal"... twenty-two is too young for this to be happening and I feel terrible. Any advice would be appreciated.