I have the exact same feeling. It seemed when I was with my ex that literally dozens of girls started looking at me for no reason and some even started conversations with me (!). I never did anything, cause at the moment I didn't want anyone but my girlfriend. But as soon as she broke up with me, it seemed that all these other girls.... vanished, just like that.
After it ended, I was desperately looking for someone that would fill the void. It's not the smartest thing to do, to immediatly look for someone else after a breakup, but back then I didn't know better. I almost got hooked up with this other girl who after some prodding let it be known I wasn't her type. It wasn't as painful this time around, and we are still friendly with each other. And alone, I could think about what had happened and WHY it had. I haven't figured it all out yet even after two years, but I think some things I understand better now.