Need some advice...

I have a question that I need some advice on bc I'm not sure how to look at it or if I'm taking it the wrong way. My fiancé and I have been knowing each other about 10 1/2 years. We've been on and off and during a time we weren't together he met someone and got her pregnant. Since then, he has done some lying and secret keeping, even cheating with his mothers child although he still denies it. Well, it's been about 3 years since we've gotten back together and he swears he's changed and only speaks with her concerning their child. We went to a park, and we were just chatting and enjoying the day, when he suddenly sees her car pull up. He like oh we're leaving and I ask him why, he says bc he doesn't want to see her. I begin to push further and asked him why would he allow her to have power over him so much so that he'd leave a public place just bc she is there. His answer is its not about that, he says he just doesn't want to be around her if it's not dealing with him going to get his child, bring him home, or just communicating about the child. So, he proceeded to move to the other side of the park, so that he wouldn't run in to her and she wouldn't see him, according to him, to avoid her asking why he wasn't watching their son play, even though he said he didn't even know he had a game. I want to believe that, but I also think that he didn't want to run into her bc I was with him, even though I believe I'm being silly bc she knows we're together, we even go places with the both of our kids and he's at age to where he tells her everything. What should I make of this? Am I wrong for thinking the worst?