Is he lying about cheating?

My boyfriend and I have been together for about 2.5 years. For about a year he’s been in the army, so we’ve been long distant that whole time. I’ve never had a reason to not trust him. We’re always open and we talk every day. For the past month I feel like we’ve been a little more distant. That could have something to do with the stress of his job and not seeing each other like a normal couple. Although we see each other every few months. I called him last night and first thing I hear is his friend clearly and loudly saying “how about that girl on tinder?” Or something like that. Two key words that stood out to me was “tinder” and “that girl”. Now his friends can be total assholes and throw jokes like that around. That’s how guys are. 🙄 My boyfriend said he was just being an asshole. I got upset and worried. Asked if he’s telling the truth. He promised to me that he’s being faithful. I already have anxiety and trust issues and I can’t stop thinking about even after he said he’s not cheating. What do you think? What are some red flags after something like that? I desperately want to believe that he’s being faithful.