She is 23 and I am 27 and we are together 3 months. It is my second relationship after the first one which was serious and lasted 5 years. I had in mind to be cool with her and not make her my main concern in my life. It was something between FWB and relationship for me.
A few days ago we went to a club with some friends of her. After a few minutes a guy came over and said hi to her. I did not know him and she introduced him to me. After 3 minutes he left and I asked her who was he because it was obvious they were not friends and her reaction was uncommon when she saw him. She told me he was just a guy she knew then I asked her straight up if they were fucking. She answered was yes but they were not officially together. She also told me he was fucking her best friend and some times together. I started getting hard and told her that and laughed. When she saw that I was cool she told me that he was also very big. I felt weird when I hear this and kept asking things (many details but it is pointless to explain here). Sometimes alcohol can make us reveal things we do not want.