I'm B 26 yr old female in california & I've been with my girlfriend (D) for 4 months... ?

Things have gotten really intense quickly, She told me she love 1 month after us going out. Things we going good but she told me she has a confession to make... She said she had phone sex last night with her ex girlfriend who lives in pennsylvania D tells me she's sorry, she love me & made a big mistake. I'm not sure on what to do cause I like her a lot... Please help me. Thank you.