Not feeling it

Lately for some reason I haven't felt like having sex with my bf. He is understanding but last time I told him I didn't feel in the mood he asked if he was doing something wrong, and he wasn't. It made me feel bad that he would think that he is the reason I don't want to have sex. I can't bring myself to tell him I simply don't wanna do it. Any ideas on how I can handle that without him feeling like its his fault?