I am not skinny either. Size 16...and was that size when I rushed. Granted, I went to a Northern school, so it could be different than down South, but PLEASE don't let your size bother you. In all honesty, would you really want to join a sorority that cares more about how you make them look with your outward appearance than whatever else it is that you have to offer? I'm sure you have plenty of other great qualities about you that will make you stand out much more than your size--and in a positive way too!
I too admit that I care about what people think about how I look--but it's all about what you wear and how you wear it. Most people don't realize how heavy I am because I wear flattering clothing that's completely in style. I suggest wearing clothes during rush that you feel comfortable and happy in--if you're not worried about how you 'look', then you'll feel more comfortable when you're talking to people. There's nothing worse than feeling insecure and having people know it!
Enjoy your new haircut! That always makes me happy