My wife thinks I'm cheating... What do I do?

I AM NOT CHEATING. Just so that's known. We have been married 12 years. Two kids 1.5 and 3. I love my wife and my kids with all of my heart. She wanted to be a stay at home, so I opened a construction company and work a decent amount. (55 hrs +/-) Pregnancy was hard on her mentally and physically. I love her and still think she is beautiful, but she has put on about 50-60 lbs since becoming a mom. And she has a hard time with it. I'm not very sexually active due to some past life stuff, so I don't often make advances and I know this upsets her. She found underwear that I'm pretty sure is hers from years ago, just lost in the back of the drawer. She swears it's not hers and kinda lost it. I get it but why would I bring home someone else's underwear, and she is always home. I let her go through my phone, texts, emails, map travels, everything I have nothing to hide. She says she dropped it, but it just feels weird now.

I love her and would never do this to her, or my kids. Is there anything I can do to make her believe me and trust me like she did before!?