I don't trust him anymore

 I have always trusted my SO, he's never done anything to make me feel that I shouldn't. But now I'm 16 weeks pregnant and for some reason I feel like I can't trust him. Nothing has happened so I'm wondering if I'm just emotional but I don't want to be stressed over it for the baby's sake. We don't live together and won't be until the end of December, I only see him 3 nights a week because he has 3 kids and needs to be home the rest of the time. (There are problems with his ex so I'm not really around the kids.) so maybe the situation mixed with being pregnant is doing it. I don't want to upset him by saying anything because he hasn't done anything to make me think I shouldn't trust him. Any ideas on how I can put these feelings to rest? I don't want to worry about it anymore