Search Results: he sent me a nude picture

My boyfriend of 2 years just recently decided to pledge a fraternity. He had to get a naked picture of a girl for one of his pledge tasks. For obvious reasons neither me or him wanted that picture to be of me (we didn't want his whole frat seeing me naked-gross) so my best friend said she'd do it. I was completely fine with that because I trust her and know she is totally not into my boyfriend and just likes doing crazy stuff like sending pics to frat guys.

Anyway I thought it was over and done with but today my boyfriend accidently let slip that he'd been recieving more nude pics from other girls. He didn't tell me about it and won't tell me who they were from so now I'm super uncomfortable. I got mad and told him that since we're in a relationship he should've at least told me that he was getting pictures other than the one I had approved. The thought of my boyfriend having naked pics of other girls on his phone makes me sick. And if they're willing to send him nude photos then they might be trying to do other things.

I'm just mad because I'm a "cool" girlfriend and my rule was always, "If they make you do something sketchy just warn me first" and the fact that he didn't tell me when it was going on makes me feel like he's hiding something more than simply a nude pic.

Also, he's refusing to talk about it. He's saying I'm being "irrational" and that we're not married and he's "doing it for a cause" so it's none of my business.

He's always been 100% faithful and this isn't like him. He's acting like nothing is wrong- buying me dinner and being sweet and I'm going fucking insane.

What do you guys think? Am I overreacting or should I be concerned?