Is my boyfriend guilty of cheating if he gets really upset when I confront him with questions about his where?

I've had previous feelings that my boyfriend of five years has cheated on me. When I ask him if he has cheated his gets really upset and defensive. I don't understand why he would get so upset he he is innocent. When he confronts me about cheating I never "blow up" about him questioning me. I need to know is this a sign of him being guilty of cheating.