Is my girlfriend cheating on me?

my girlfriend has an online boyfriend..she alwaz chats with him..and she gets angry whenever im home at around 10 in the night coz thats usually the time when they chat...she keeps sending him offline messages like please come online tomoro at this time and that time....but once i caught her and she says she have never seen that person and instead of saying sorry for wat she did,,,she started shouting at me...and i still see that she kips chatting with him..but never in front of me...but i saw her chat sessions with him...she have used al those beautiful words that she never use with me thesedaz....please advise me wat should i do now? i really want to get rid of it...i want to leave her for wat all she have done..but still im feeling pity on her and she knows that..thatsy i think she is taking advantage of my love and my patience..please advise wats the right thing to do now...i will really appreciate ur help....thanks