Having a bad feeling...

Okay so I'm 21 weeks pregnant with a baby boy and not everything between my boyfriend and I has been just peaches and cream. We don't live together and been fighting a lot lately... there's this girl at his job that he's been dropping her off and texting her but he won't show me his phone or anything like that. I'm having a really bad feeling that he's cheating on me but he looked me dead in the eye and told me he isn't and that he isn't like that... I don't know what I should do... any advice?