How could he do this?

My fiance and I have been together for 2 years now, he has helped me raise my oldest son from 8 months old and 11 months ago I had another little boy with him, and I'm now pregnant with our 3rd little boy....here lately my fiance has been growing more and more distant from me. He does work but he'll be out till 3, 4 or sometimes even 5 in the morning and say he worked the whole time, one night he got off work at 8:30p but didnt' come home till 1:30p and the reason I know that is because my brother in law saw him and last night he got off at 1:30a but didnt make it home till 3a...he says he hit a skunk and had to stop at a gas station cuz it was makin him sick...and then he got up at 9 this morning and left again. He is the only one working so I dont want to jump to conclusions but I'm really startin to wonder...a girl he works with emailed me on fb the other day and told me that he'd been flirtin with her and tryin to get her number....I dont know what to think....what should I do? What does this sound like to you?