Iam a 30 year old guy and I just found out that my long time G/F is cheating on me,she dosent know that I know?

I just found out that my long time G/F that tells me she loves and adores me is cheating on me,she dosent know that I know,we just broke up lastnight but not for this reason,if she calls back do u think i should tell her that shes a tramp? or should i play it cool and act as if i dont know anything? the bad part is that the other guy was at my party like 3 nights ago,she had invited him ! I feel like such a idiot.