My girlfriend and I met through work. She recently broke up with me for understandable reasons. I had been thinking of breaking it off, stopped going over to her house, and stopped making too much of an effort. She (rightfully in my mind) broke up with me. Well, the next couple of weeks she would come over and play sims and drink with me until we got drunk and fucked. Also we agreed to still be "best friends". This happened at least three times in two weeks. We are still very good friends and work together too. Well a couple days ago the employees went out to a bar for a little r&amp;r and after the bar closed the closest 5 of us went back to one of the employees houses. Let's call this guy Rick. We all went to Rick's house late late at night. Maybe 2 or 3 in the morning.  I was getting tired and decided to uber home. The ex stopped me and asked if I was okay. I said yeah; just drank too much and am going home. She said okay and I left alone. Well the next morning she invites me to breakfast and I agree. We meet at a taco place and she reveals that she slept with Rick after I left. I tell her it's all good and she is her own person and she can do whatever she wants. Rick also asks me if its okay. Well I am extremely upset and I dont think this is okay at all. I'm mad at both parties and I don't know what to do. Am I the asshole for wanting them to cut it out, or can they date because I broke up with her? 

TLDR: girlfriend and I break up, she fucks one of my only co-workers/good friend. I'm upset, but should I be allowed to be? 