Not sure what to do. Updated

Ladies I was looking at my SO's phone the other day and saw that on Snapchat he has a 28 day streak with a girl. We went to school with her and they were friends before we started dating. I don't trust this girl because she is just a flirty person, and to me she has no boundaries. Should I confront her and tell her to stop sending him snapchats or should I tell him that I'm uncomfortable with them talking so often? I saw this about a week ago and I have been in a horrible mood all week, especially when I hear his phone go off because I automatically think it's her. I'm just not sure what I should do and I hate feeling this way. He has no idea why I'm so mad and I think it's a little unfair to be so mad at him if he doesn't even know what he's doing wrong. Any advice appreciated.

UPDATE:

I talked to him shortly after posting this. He got mad at me and basically said I don’t trust him. I trust him but I just don’t trust her at all. He said I had no right to get mad when I occasionally talk to guys. However he did say he would stop talking to her. He lied about that because they are still talking. What should I do? I am really mad and also hurt at the same time. I don’t want this stupid girl to ruin a 3 year relationship because she doesn’t understand boundaries. I know it’s not all her fault though. Please give me some advice