She is my only girlfriend i had and we've been dating for 3 months. Never got into an argument but today she caught me looking at this other girl for a good 5 seconds and now she is a bit mad and won't talk to me. My friend said it's ok and that she will get over it, is this true?
Yes she will get over it. You should tell her to be more confident in herself and in you, because at the end of the day you don't have to be with her but you are. No matter how many chicks your digging you have your emotions invested in her, no one else.