what should I do? I don't know who else to go to

my best friend from where I used to live came to visit me this week. my girlfriend wanted to meet him and I was a little nervous about that, I didn’t really know why. but I figured nothing bad would happen because of it. but all day all my girlfriend did was flirt with him and he did the same with her. all day they did this right in front of me. she never seemed to even acknowledge that I was there. we were helping her family take down their chrismas lights and afterwards, they were asking if he was coming out next year to help again, not one bit of apriciation went to me. like my girlfriend they didn’t seem to care. I was really hurt by everything they did today and really don’t know who to talk to about this because my girlfriend doesn’t seem to get understand why im upset and my best friend is just saying “im sorry, I didn’t realize, I was just being me” I’ve known him almost his entire life, that wasn’t him. the people who I usually go to for advice on this are the ones involved so I don’t know who to go to for help. what should I do here? am I just over-reacting? and I don’t even know what to say to them, because I’ve alway been the strong guy, who can’t ever be hurt emotionally. but this time I can’t help it, this really hurt me.