Had the worst nightmare in my life?

I dreamed about my former crush was with a guy (a friend of mine). They were flirting, playing around a little and i was right there seeing it all happened. After a few minutes, i broke into tears and ran back inside my home and screamed and yelling "Why!?" on my pillow. Before this dream happen, she told me that she doesn't really wanted to be in a relationship with anyone and i was ok with it and want to remain as friends until we just stopped talking after that. As of them it just blew my head wide open and can't even sleep. Help?