Ex girlfriend problems. Boy do I have a situation.

I am 21 years old as well as my Ex Girlfriend. We dated for 2 years and recently broke up a month ago. And over the course of the last month I sucked up my pride and tried to do everything in my power to get her back. Nothing was working. Even my Ex said that we could never be in a relationship again and sex was defiantly not an option anymore. so eventually i gave her the space she wanted and stopped putting effort into a dead relationship. (we both still care about each other very much). And as we all know one of the best ways to get over a girl is to get with another. And that is exactly what i did. A couple days ago I went out with some of my boys and we picked up a few girls for the night. waking up the next morning i realized my neck looked like it got torn apart by an animal, and that was the day i was going to hang out with my ex "as friends". So upon meeting up with my Ex, she noticed the marking on my neck and shoulders and called me out about it. She then decided not to talk to me for a couple days. But later that week we started hanging out again and we literally just started having sex out of no were!!! So now i'm sitting here confused as hell. My question is "why is my ex all of a sudden so attached to me again? is it because, i made her jealous? "