Can you be sure they were talking about you? Did they use your first and last names? How well do you know these girls? Do you ever hang out with them at school, or during breaks. Have you talked to them this summer? Did your friendships with them die out? Have they ever bullied or belittled you before?
You're going to ask yourself those above questions, if you want to get to the bottom of why you saw your name being used in a conversation that you weren't even a part of. It's possible they were acting like immature mean girls, saying stupid things about other girls, and your name came up as a butt of one of their digs. Hopefully that's not the case. Bit if they were being bit©hes, they aren't worth your time. It takes bullies like them a long time to mature into decent people, so try to ignore them. Instead of following them on Twitter, follow your best friends, & family. Spend some real time with them just hanging out or find something fun to do.