Hello and welcome. First of all how old are you, you sound pretty young?
Every parent has nightmares about some of the friends their children hang out with. It is just the nature of parenting. Something has triggered your girlfriend's Mother to have concerns. Do you know what the something is?
I think for now taking a big breath and letting things cool down a bit would serve you well. You may be reading much more into the situation then is really happening.
Teenagers deal with stress in just about every facet of their lives: academics, sports, social situations, family life, money matters, even work. There is a great book out that you may want to try to find as it may help you in learning how to deal with stress, Don't Sweat the Small Stuff for Teens, Simple Ways to Keep Your Cool in Stressful Times by Richard Carlson, Ph.D.
Again welcome to HealingWell and I am glad to meet you,