I don't know...

So my boyfriend and I have been having more arguments than usual and I will admit sometimes I am unreasonable and I'm working on it.. Anyways! This morning my bf was texting me and it was kinda dry like one or two word answers... At one point I stopped texting him because I was in a lecture and there were only a few of us I there so I didn't want to be rude and my prof was talking about something interesting. Next thing I know, my phone is ringing and it's my bf. Obviously I didn't answer it and there were only 10 or so mins left in the lecture so I waited... Once I got out of class I listened to my voicemail and it's him his exact words are "Hi it's Wes. We need to talk so call me. Bye." He NEVER starts a message like that! So I call him and he says we need to talk but in person so at this point I'm thinking the worst... So an hour later he meets me at school (we go to the same college and he had class in a few hours as did I). Oh! He also texted me before he got there asking if I knew a random number but I was like oh whatever no I don't. So he sits me down and says "I got this text this morning..." He gives me his phone and it's a random number. They say something along the lines of this "hi is this Alexa's boyfriend? (Bf replies yes and questions who it is) Well I thought I would let you know that she hasn't been all that honest... (Bf is like umm okay how?) You aren't the only one she's with and I'm sorry to be telling you this but she's been doing other things with them too..." They wouldn't admit who they were as they only said they were a "friend of a friend". We've asked all our friends and obviously it only had to be someone with his number. We tried calling it and there's no voicemail and when we tried doing the yellow pages reverse lookup, no results were found. Also, just giving you a bit more background info to this; I'm in college full-time, work two jobs so when I'm not at any of those I'm either home doing homework or with my boyfriend. I do mainly have guy friends because I can't stand drama with girls but haven't seen them in a while and my bf knows but has met all of them. He has been cheated on before and knows I would never ever cheat on him or anyone EVER! We are a very happy, easy-going couple and most people like us (I think or at least I'd like to think) so I have no idea who would to try to break us up. My question is what would you do if you were in my position? Do you guys have similar stories?