So their relationship ended, (we're not told how, but he wasn't happy about it) and he decided to do his mandatory 2 years in the military.
Assuming that she ended it, she's probably regretting it (maybe another relationship ended, or it's possible that she never got over it either, she's at least declared that she doesn't want to date anyone now.)
Then she hears ex is back, single and also not wanting to start a relationship. So she starts getting feelings again, but is either not sure or too proud to admit it.
Enter new girl who is slowly taking her place in the guy's world. (Very quickly, already "marking" him with perfume and sparking his interest in photography again)
So her anger is understandable, NOT acceptable, but understandable.