I'm worried about him cheating.

My boyfriend and I have been together for 2 years. He swears he'd never cheat on me. But because I've been cheated on so many times, it's what I assume every time something wrong happens. But lately, he's been hiding his phone, turning it away from me when texting and won't even let me touch it.  And a few days ago, I noticed muddy footprints on the carpet of his truck, I put my shoe up to it, because his sisters and mom have bigger feet than I do… and the footprints were smaller than mine. Every time I try approaching the subject, it turns into a fight. What do I do?