...please explain

So I just turned 29 weeks.. 
and my boyfriend is extremely upset and gets mad when I deny him for sex. We haven't had sex in a week, and to be honest I have no intention or want to. NOT that he doesn't attract me because he does, & I love him but I have no desire and it's getting more uncomfortable. He thinks I'm cheating on him and wonders why I never wanna do it. 
Any advice on what to do??? :( 
And yes , I've tried to tell him that it's uncomfortable.. that doesn't work. :/ 
I'm lost and not sure what to do. He gets so mad and upset at me