JACOB, it's simple: either she's just broken up with someone, she's lonely, she's horny, or she's bored. Girls like to go back to what's familiar because they want something "now." So they hit up ex's because they feel most of the work is done. It should be easier to get it in. IDK about you, but messing with ex's is easy and fun. Then you get reminded of why you broke up and it's easier to stop talking again. There's pro's to messing with ex's. LOL.
Oh, yeah, ulterior motives aside, it's possible that she just misses you. Maybe. I just assume the worst about women until proven otherwise.