I need help im worried my girlfriend might be cheating?

Me and this girl we are boat marine polies. Me and her joke around alot neither of us like being clingy so were not. But i guess some times i can act like a real dick sometimes even though i dont mean to but so can she. And she always takes forever to reply when we do talk. (Always on snapchat) and sometimes she will open it and not reply. But we have this marine group chat and and we were going to hang out and theres this guy named zach also marine polie and shes always asking him to come and i said like an *** hole you would like that wouldnt you and it made her mad then the zach guy left the group and she kept adding him back. And she likes alot of his stuff on insta and hardly anything i post i mean she tells me she likes me and she does want to be with me she just told me she likes me alot. And she also knew this guy alot longer than me i guess they have been friends for a long time. Idk if im paranoid or what