Problems with an ex friend?

Sorry this is gonna be a bit long. I am a sopre in college and over last year, I became good friends with a group of people. One girl(lets call her Mary for the sake of things) was the one that I thought I was closest to. When we got back to school this year, she and 3 of the other girls we hang out with were all in a quad together, and me and 2 other girls who are friends with them, lived down the hall. Immediately I noticed that Mary was acting different then the way she was last year. She is very child like at heart, but never went to far with it to where it got on peoples nerves. Now though she was still acting child like but in a bad way. She was being bratty and very selfish at times. We went to the mall a few weeks ago and began to repeat the same phrase over and over again and wouldn't stop and it started getting on our one friends nerves real bad, and when I asked her to stop she snapped at me and said it was her car, and that she could do what she wanted in it and if we didn't like it get out and walk back. I held my tongue and let it go, thinking not to start anything. But then a few days latter we were sitting in the quad that they live in watch tv. At first we were watching House but then one girl( call her Sam) changed the channel during commercials, and Mary saw Dancing With the Stars was on and asked to keep on that channel. We did but when commercials came on Sam went to go change the channel and Mary immediately stopped her, took the remote from her, and was playing keep away with it. I saw this and thought, let me just change the channel manually and be done with it, its just one channel down. The minute I changed the channel though, Mary started angrily telling me to change it back and sit the hell down. She was talking down to me like you would to a dog that just did something really bad. She finally told me to sit my a** down or get out of her room. In a moment of anger I told her to go f**k herself, grabbed my things and left. There was only one other person in the room who lived there, and she said nothing. The other to were out at the moment, and it was just me, Mary, Sam, and this one other girl who lives in the hall. Since then, Mary has ignored my existence and has made me feel unwelcome when ever I'm around. She defriended and blocked me on facebook, she treats me like I'm invisible, and trys to justify it by saying she is doing this because of things that happened last year, that I apologized for and don't warrant this kind of treatment. And now its worse, cause when ever one of her roommates does something with me I feel like she retaliates against them because of it. Me and her one roommate who wasn't there for the fight wanted to watch something Tuesday. When we talked about it while she was in the room, she reacted by putting a movie in an hour before the show we wanted to watch was to be on just so we couldn't watch it there. Then I left to go somewhere for a minute. When I came back, the door was closed and lock. I knocked but Mary kept saying she didn't see anyone through the pep hole and when her roommate told her it was me, she then said she wasn't gonna open the door at all then. Needless to say, me and her roommate watched the show in my room. I stood up to her and she is treating me like crap, and even though 2 people have talked to her, no one seems to really have my back on this. They all keep saying that she is in the wrong and acting childish, and I did nothing wrong, but that seems to be the only support they're giving me. Part of me wants them to stopping being her friend, but that's selfish and I would never ask that of them, but I don't feel like they're there for me. And feel bad when Mary is mean to them cause they are doing something with me. I'm thinking of asking an R.A. or counselor for help, but I don't want to cause anymore trouble then I already have. Can someone help me figure out what the right thing to do is, I feel so lost right now.