Is she in a rebound relationship?

Basically me and my ex where really in love like always seeing each other and all that for 2 years then she suddenly lost her feelings for me and asked for a break I failed at that and kept talking (bad idea!) then she ended it a week later. So after the break up I was still trying to get her back and it only made things worse I would argue with her (first proper argument we ever had over the 2 years) then I decided I didn't want to see or talk to her to help me heal so I deleted her from facebook and what not and we didn't talk for a month. Now we have remade contact 2 monthsish after the break up and she has found a new man and have been together for nearly 3 weeks now and she has already had sex with him - when she dated me she was a virgin and VERY innocent and insecure with her body and being sexual. So she is now always round his house and pretty much acting like she was when she was with me. She still calls me by the nickname she gave me and sometimes reminisces about memories from the relationship. So basically is she in a rebound? and how could I use this to win her back? thanks x