Should I be angry about my gfs sexual past?

Iv been with my gf for a year and a half and I love her but over a few months ago she told me she had oral sex with one of her exes and it made me feel like crap. For the past several months I have just been bottling up my emotions and every now and then I have been seeing reoccurring images of her and her ex. I know that this was a while ago but it just pains me to think about this. Is there any way to help get past this?any advice would help...