My best friends are leaving me out?

Okay so next week we are having spirit week and one of the days is twin day . k so me and 3 other girls [4 total] were going to dress up as the sesame characters . the only reason why im in it is bc they know i heard them talking about it -__- . they are already being elmo, cookie monster, and big bird , and what am i , the oscar the garbage man . GREAT , like i was all excited saying we can go to the mall in MY car to get the shirts/hats , but i feel like i am unwanted. those are the only normal ppl in my class and my other friend is doing it with someone else . idk what to do ! i really wanted to dress up with them , BUT NO . what should i do ? am i over reacting ? i just thought it would be nice of them to actually include me . and not being consided, but 2 of the girls, like me better than the other girl who was originally in it . so wth's up with them ? pleaase hellp !!?? thanks :]