What should I do?

ok this is going to take a while to explain but just hang with me I need to know what you think. Ok me and my amazing girlfriend have been going out for about a year and 4 months now and I love her with all of my heart and I would do anything for her, but latly things have been hapaning. Last Friday we had an amazing night just hanging out and stuff, but the next Monday she just completly egnored me and started flirting with one of my friends. She clames that she isnt but I shure does look like it. And we have been mad at eachother form then on. So what should I do to fix this. And don’t say just dump her or any of that crap because I don’t want to hear any of that. I just want to know what to say and how to move on and continue this relationship with her