Possible cheating boyfriend!?

My boifriend was a computer illiterate and I taught him the basics when we got our home pc. Now he is on facebook all hours just adding random girls. In the beginning i didnt really care because it was harmless him and his cousin were just being childish to see how many hot girls they can manage make as their friends. Now im actually seeing meesages that he is sending girls telling them all kind of crap about them and also exchanging phone numbers. despite the fact that i answer his phone as well and hes never on the phone it still upsets me because I feel everytime i step away I know hes going to be up to something. Its not like i was even snooping on him we both have our emails and paswords saved on our pc so we never type them in. I just opend the facebook browser one day and was in his facebook because he didnt log out. Am i just freaking out for no reason or would another one of you guys be upset at that too? Hes somewhat just totally disghusted me in some way because just way he is so desperate in his messages is gross. I mean we have a baby together and another one on the way. Am i just being hormonal? I havent confronted him because I just dont want anymore unessecery stress to deal with. What do i honestly do???/