My 3 Best Friends in the World Just Started Ignoring Me...What Should I Do?

Well this begun about a month ago. The 4 of us were so close, we knew everything about each other and we never had fights. Now i'm 13 years old. They act like i'm so remote. Especially my best friend mandy. We were always really annoyed with one of the other 3 lexie. She got really annoyed because she always copies her. Now all of the sudden she's magically just hanging out with her. I sit next to mandy at lunch and when lexie gets there she makes me move to a different table. my other friend Laura helps. We walk to the bus ramp and they will just walk right past me. Why? Should i just let go and look for new friends?