Am I Being Paranoid Or Is He Cheating?

My husband and I have had some issues in our marriage. He cheated on me a few years back. I found the panties in his car. We have since been trying to work through it with couples and individual therapy. Last Friday he went to his doctors appointment for a check up and he came back acting change and uncomfortable. I made him dinner and he just randomly said "I'm gonna lose the damn weight". He is extremely over weight and I've been trying to get him to eat healthy and exercise for years now. Why all of a sudden the change? He put himself on a diet and started going on jogs. Well trying too. He's really out of shape so he's gonna have to take it slowly. Could he be trying to impress someone. We have come so far in our marriage. I can't go through the cheating again.