I don't understand

I don't understand why my fiancé does not believe me when I tell him that the MS makes me tired. He thinks I use it as an excuse and that I'm lying. I told him about me having MS the first day we met. I told him to research it before he decided if he wanted to pursue anything with me. He said he did and he wanted to. So why now is this such a problem? 

We were just at TGI Fridays and had a few beers. I got tired because the week before we were getting up early and driving an hour away to take care of his mom who just had surgery. He says I just want to lay on the couch. That's so not true!! I'm just exhausted!! Maybe I should leave. I just don't know.....