Please help I’m at a complete loss here😔

So I just recently found out my fiancé had slept with somebody else back in January. During that time he and I had “broken up” and we got back together in February. When we got back together I asked him had anything happened had he seen anybody else. He said no immediately. Well that wasn’t the case and now i don’t know how to react. I would have forgiven him immediately if he would have came clean when I asked him but now idk why to do. Also we just had a baby in October 2017 and I’m currently 15 weeks pregnant. Idk if I should stay or just leave him. I’m going to talk to him today about how I feel. We haven’t talked because I had a health scare that same week so I just push that to the side. If you guys could give me some advice on what to say to him, what are the things that I should ask him about what happen between him and that girl. I’m just completely lost.

When I say “broken up” we just had needed some space from each other we were still living together , still in the same bed, still a “family”. The only reason I found out was because I found condoms in his car while I was cleaning it out to move everything to his new car. We haven’t used a condom in over 4 years