nothing is wrong with you, you invested emotionally in this girl, and part of your male brain circuitry is to be territorial/possessive over your sexual partners. other males having sexual access to a girl tied into this emotional circuitry trips off negative feelings - then factor in that our existing brain circuitry isnt geared for large, sexually progressive societies where switching partners is socially acceptable, and you feel undermined/confused/tormented ans your "KILL THE COMPETING MALE" circuitry goes to war with your rational "thinking" brain
plus without thinking about it, lots of people subconciously judge their own value by looking at the partner theyre with (the better she is, the better you too must be) and other males easily having access to that woman undermines your self worth, and makes you feel insecure
oh. the reason she told you that shes doing these guys is because of the above - shes trying to devalue you in her own mind (showing her greater value through ability to hurt you) so that she doesnt feel as bad because of the breakup. women are designed to feel a lot better when the upgrade their man than when they downgrade - an easy way to feel like theyve upgraded is simply to devalue you in the first place. my major ex systematically went through her memories of me and i quote "focussed on all the bad things" so as to devalue me and get over me faster, feeling good about getting with guys that i VASTLY outclass.
if your ex had any understanding of what shes doing, she would have kept her new BFs on the low, since shes building some serious barriers to you two getting back together, which is dumb. (my latest fave ex is doing the same thing. shes also 19. bitch! biiiiitch!! lol)