Relationship tension

Lately I've been experiencing something that troubles me very deeply. My boyfriend and I have always been very intimate and we love each other so much but lately when we try to be intimate I am not relaxed or comfortable at all!! This worries me because it's never happened before just the last few times. I really trust and love him. I know he's the one for me but I just don't know what to do!! Please help me! I need advice!