Sexless relationships problems..

I'm not sure what it is. But ever since I had my baby I am completely and utterly uninterested in sex with my fiancee. (We've been together over 4 years now and our baby is a little over 15 months.)

We have had so many fights and arguments about it, but I just cant seem to put sex on my priority list. For some reason it makes me uncomfortable. Sometimes I dont even like to be touched or kissed. I feel like there is something wrong with me. And the pressure coming from him is driving me insane. He talks about it daily. Please help if you can...