Should we stay together even though she kissed her ex twice while I was away on business?

I was away for a couple weeks on a business trip and my girlfriend was spending a lot time with her ex. I told her that it wasn't a good idea because he still has feelings for her. But she didn't listen to me and they hung out 24/7. The day before I came back she informed me that she was falling for him and old feelings were coming back and he kissed her twice. She said that she wanted to be with me and when I came back up there she wanted to marry me. I don't know if I can forgive and forget about this..I don't trust her and I think something more happened between them..I love this girl with all my heart but I don't know if I can trust her again..