Help!!!

My fiancé has told me multiple times that he dislikes a certain girl.. I strongly dislike her as well.. But I had just checked his Facebook, only cuz I wanted to see what she sent him. She messaged him asking for me.. Idc about that but the further I went back into the conversation I realized that he was flirting with her and all that... It just upsets me cuz I asked about it and he totally lied to my face... He has never lied to me before but I think he has been more and more recently... Idk what to do😡💔