Conflicted. Should I believe him?

Me and my bf have been together for almost 6 years and we recently had a baby together. Right after our baby was born he started being weird. Very distant and always picking fights with me. It went on for about 2 months and finally I felt the urge to check his phone. I found a girls phone number and he doesn’t talk to any girls at all he never really has or cared to have female friends so I was quite weirded out and the second I saw the name I got a weird feeling and he started acting weird yelling at me saying he was supposedly going to leave me a year ago and that she’s just some girl from work and he needed her help and that’s why he had her number. But him acting weird still was throwing me off. Anyways I checked the phone bill and he wasn’t lying when he said he never texted her but then a few weeks later I saw that her texted her and deleted the messages and he said it was bc he knew I would check the bill then he says he rarely spoken to her but when I came to pick him up unexpectedly I saw him walking out with her and when he quit she texted him and we got in a fight about it but deleted the messages again. And idk if I’m just being irrational but it’s been like 4 months now and I can’t stop thinking about this. I just had this feeling he liked her behind my back and I look dumb ass hell being that gf that doesn’t know.

Now that he doesn’t work there anymore he’s back to normal he blocked her number and social media bc he says I’m obviously more important and he doesn’t want that drama in our relationship. And he said he has never been interested in anyone else and he’s not that type of person. But my mind keeps playing tricks on me.

Any thoughts? Or advice to move on ?