my g[f] 20 ran into an ex [m]23 and is putting [m]e 24 on the backburner it seems

i have been dating my girlfriend for almost 3 months now, and things have been going great. we recently went back home for thanksgiving (we came from separate towns) and see ran into her first serious boyfriend, her ex, while she was home. since then, she has been very distant and not telling me anything and going to her friends for more advice.

she broke up with him years ago because he didnt have his life together. shes told me that it was the hardest decision shes ever had to make. he told her that he had recently gotten dumped from a 3.5 year relationship and has been taking it pretty hard (his ex cheated on him for the last 2 years of their relationship). this is the first time they have really talked since they broke up almost 4 years ago and since we got back from holiday, she has been talking to him non-stop and her old feelings are coming back up and she is "torn in two" (her words). she barely talks to me right now when we are together, her sex drive has gone down the tubes, and im stuck fearing the worst but hoping for the best.

she makes me happier than i have ever been, and i dont want to see this relationship suffer because of someone from her past suddenly popping back up. if anyone has any advice, id gladly appreciate it since i dont have anyone down here that i can go to.

tl;dr: old feelings from an ex are coming back up in my gf and im getting left in the dark.