Roommate's exclude me a lot?

I have never really had this problem before so I don't know whether I am overreacting. This is my 3rd time living with a new group of people, and I've always made friends very easily. I'm in a new program where it seems the majority of people in the program get along with me as well.

However, I was placed in a living situation with 2 girls, whom I got along with just fine at orientation. They are both around the same age (about 3-5 years older but its hardly noticeable), they're both from out of state, they both smoke, etc. So they have a lot in common that I do not. However, outside of those things, their personalities are polar opposites and they initially didn't really like each other. They even got into a huge fight recently.

So this is why I can't figure out why, when they're not fighting, they seem to buddy up and exclude me. They'll blatantly ask each other to go place (store, fast food, etc) in front of me, and not invite me. They'll have a good time conversing with each other, and then stop talking when I come in the room. They'll let each other know when they're going to run some errands, and not tell me. They also talk a lot of crap about people from my state right in front of me. One girl almost always treats me like this, while the other will act like my best friend sometimes, and then totally ignore me the next.

I'm not sure what it is as I've never encountered this sort of bi polar behavior before, but it seems like they hate each other, and at the same time really crave the others' approval and don't give a sh*t about what I think because I'm the "nice" one. At the same time they've both mentioned how jealous they are that I"m getting better grades, and that people in the program really like me (and honestly don't really like them). What's the deal here? And most importantly, what do I do!? I'm starting to feel like I look desperate when I go out of my way to talk or spend time with them.
Roommate's exclude me a lot?
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