I wouldn’t be quick to make judgment just yet. They could have been whispering about something that didn’t have anything to do with you. If you feel unsettled over it, kindly confront them. Say, “I felt uncomfortable when I presented my idea and you whispered. I don’t want to point fingers but I felt like you were talking about me.” If she denies saying anything (which she probably will) use this opportunity to bring up her past behavior. You could say something along the lines of “I only ask because lately you have been really attitude-y and hurtful when it comes to dancing and it's been making me feel really bad.” Be sure that you don’t say things like “You have been acting you’re better than everyone else” or “You’ve been catty.” This will only encourage her to be mean and it will close her off to a mature discussion.
If she continues to be rude to you, ignore her until she gets her act together, but don’t gossip about her. By doing that, you are stooping to her level and you’re better than that. More importantly, don’t feel bad about where you go for dance lessons; what matters is your talent, not the name of the place you go to learn. Keep your head up and be proud of yourself. Don’t let her ruin something you really love doing and don’t let her get the best of you.