Worried

I feel disgusted with myself lately everytime me and my SO do the nasty.. And I don't know why. I get feelings of disgust with myself and I feel angry and frustrated and I want to cry. I don't understand? Before we have sex I'm turned on and in the mood but as soon as we start having sex, I'm turned off and grossed out. We've been together for about a year now and this is totally not normal. And insights would be great