Why do I feel guilty!?

Dated this guy for 3 years then he dumped mw 2 weeks before Christmas 2015. I assumed it was over right? Hasn't spoken since (with the exception of arranging for me to pick up my stuff from his place) but we haven't spoken otherwise.

Afterwards (middle of February towards the end of February) I started fooling around with an old friend who's really cool. No strings attached, we've had sex 3 times. Anyways. Currently I am with a small group of friends, including this guy and 4 other people. I'm sharing a room with him I like it it's cool! And then I get a text from the ex of 3 years! Asking me how I've been. I suddenly felt guilty. I mean I had once promised this man that I'd love him forever and here I was laying beside this guy friend! Why do I feel guilty? Have I moved on too quick? Help