Whats do i do if my girl is up to no good like trying to cheat?

well i ben kinda keeping a eye out on my girl cause she ben on facebook talking to her Xe's and trust me them ben talking for a while now and then now its this other guy now at one point she ben telling this other guy how hes still fine and blow him a kiss and now i wanna no what to do cause her neighborhood reunion is to night and she just pop it up on me and she dose not want me to go to it with her she wanna go alone and i no for a fact the guys are Ghana be there so now what do i do should i go with her or stop her from going ... man its really sucks i ben with her for about 12years and two kids and man im really sadden by it this is not a good feeling ,,,,!!!! i need help please ,,,, dnt no what to do !!!!!!