That is a pretty terrible thing to do to a person. If you were strung out on heroin, drinking yourself to death, or smoking 4 packs a day, that would be different, but an intervention for weed? Come on, thats just plain dumb. If they had a problem with it, they should have been honest and talked to you straight, not trick you like this.

Now, here is my advice, dump your girlfriend. If she is willing to betray you like this, there is no telling what else she can do. As for your parents, be mad at them for awhile, but after a few day, call them up and try to have a rational discussion and work everything out. Being estranged from your parents has to be a horrible position and I wouldn't want it to happen to anyone. Those people brought you into this world, so don't be mad at them forever. Just tell them that what they did was wrong, and hopefully you can work things out.