Bye Bye Sex Life?

I'm really sad & embarrassed to post this. My boyfriend & I used to have sex non-stop. Obviously, we're pregnant with our rainbow & are so excited! Our sex was amazing we both couldn't get enough! But now, I'm 4 months pregnant & I think I look pretty amazing. Didn't let myself go, I think I look the best I've ever looked! (New perky C cups, yay). 
But my boyfriend's sex drive has diminished. I can't help but think it's me. I wanted to have sex with him last night & finally just started crying & asked him what went wrong. He feels really bad & says he has no idea, he just is never in the mood anymore. & he's frustrated. He says he's lucky he even got me in the first place & if anything, I'm even sexier now that I'm having his baby. Then what is it? Is this common for first time fathers-to-be? In relationships in general? Is it because I'm pregnant? We literally have the perfect relationship, that man takes care of me & worships me. This is our first "bump in the road."
Thank you for reading :(