Feeling betrayed and hurt by my friend?

I have no classes with any of my friends (group from a biology class we had together) and so far have only seen two of them. One of my friends is part of a sorority and invited out this girl who is my friend and her friend to a club. She has not invited me at all or even gave it any mention. I was extremely hurt when my friend told me this and hoping that maybe my other friend has not had a chance to invite me (though she also posted this on fb). I do have few friends and a pretty low self esteem so I'm thinking that my friend will not invite me cause I will embarrass her in front of her sorority, but I feel like she wouldn't this to me because she has helped me a lot. What do I do? Am I allowed to feel hurt? Have friends done this to you and your relationship with them has stayed the same? is there an ugly truth behind this? She just called me a few minutes ago with no mention about the night out.