My best friend has a girlfriend and I think that she hates me...?

My best friend got a girlfriend last week.....She is our classmate and I think she does not like me being around my friend anymore.... My best friend and his girlfriend are the top rankers of the class while I lag behind a little....I think that is one of the reasons she hates me....She is I think selfish and always makes friends with the top rankers....I feel that she is making my friend drift away from me....He now many a times spends time with her group which is full of drama queens and some boys which she does not know are bad......I am saddened by this attitude of hers towards me...Please help?