Need a little advise?

My gf and me are in a LDR and we broke up about a month ago and got back together we've been dating for 4 days and she told me when we broke up her and this other guy were talking about doing stuff together (SEXUAL) but never did so today she told me shes going to hang out with him at this rodeo thing and that her cousin will be there with her and idk what to do? also he doesn't know that shes dating