My friends did a complete 180 on me?

I am a 21 year old senior up away from home for college. I first went to a university right by my house, but then transferred away. This is gonna sound silly, but it is really bugging me. Last semester, I met three awesome girls who became my closest friends. They are in sororities, but it never bothered them that I was not in a fraternity. We would always go out together. I never thought of dating them, I saw them as sisters. They always came to me for help, and always asked me to hang out, I never had to ask them. Fraternity rush came around, and they tried to get me to come out, but since I was a senior, I didnt see a point to it, and I had already done the whole frat thing back at my home, and I had a bad experience. I explained to them everything, didnt leave anything out. At first they seemed cool with it, but a day later they acted strange, like they didnt want to have anything to do with me. It sucks, cause they were my best friends, and now I'm feeling lonely as hell. The only thing I got from them is that they wanted their space. I asked them what I did, they would never answer, or just ignore me. I dont know what to do, I know its stupid, but I invested so much into those friendships, and I don't want to lose them.