Is my boyfriend cheating on me?

I've been in a long distance relationship for three years (YIKES!)....All of a sudden I get this awful gut feeling. I checked my bf's facebook inbox (I know this is wrong) and saw that he asked 2 girls for their numbers and invited one to go out with he & his friends and told her to bring her friends on a few occasions. He has changed since he decided to join a graduate chapter of a fraternity. (Black frats can do this) It's like he's in college again! One day, I asked him if he felt it was wrong for people in a relationship to collect numbers....he said yes. I also asked him if he has asked any women for their numbers or called other girls....He denied it of course and acted furious because I told him I didn't trust him. What should I do? I have information that I shouldn't have because I was nosy, but I know he's lying to me and probably cheating. HELP! (Sorry this was so long.)