Need Advice

Hey All, Need your help on this situation. I have been with my girlfriend for 3 years and love her to death and not sure how I let this happen. I am engaged and we are getting married soon. I was at the bar a few weekends ago with a couple buddies, I rarley go to the bars but i used to go when I was single alot. I could never think of cheating on my girlfriend in a million years cause I lover her alot and thats not the type of guy i am.. I would rather break it off then mess her life up. As i was at the club with some buddies, we got a lot of drinks in us and thats were the trouble started. It was the type of night that led to blacking out/passing out at the end. It was not till the next day which really caused the concern because I didnt realize what was goin on the previous night and it went so quickly that it just went on. As i look back on that night, a girl was dancing with my buddy and started to make her way over to me, next thing i knew we where dancing with her and she started rubbing me on the dance floor. I dont remember all of it and it makes me sick. I dont remember telling her to stop but i also dont remember telling her i want more and acting on thise actions. I know I didnt touch her or kiss her at any point and the dancing/touching was maybe 5-10mins? I had a buddy there that said nothing else went on and I did nothing wrong in terms of goin after her etc. She ended up goin home with some other guy and i was not with her at the end of the night. I dont remember leaving her but they all said that I ended up leaving and my buddy just continued to dance with her. I cant seem to wrap my head around this and need some advice.. Should i tell her? How bad is this situation and is this worth breaking/ending an engagements over?