Being nice comes with a price. You get the same treatment as everyone else, people are always going to expect you to be nice to them, no matter what, they don't realize that they've got something to lose.
Some people don't realize what they have until it's gone... I'm not going to tell you to find new people to hang out with, because if that's the "group" you are in I know it's hard to get out of... But when they try to act rude after you are sweet, you need to let them know what they are doing and tell them you aren't going to take that from them