My Knees Hurt. Help me?

I dance, and my knee caps are loose, so my orthopedic doctor suggested I do squats, because I "possibly have an imbalance of muscles on each side of my knee." So every time I'm doing plies at the barre, I can't do a grand, only about half of a demi. My knees also make a popping noise when I get up, and can pop out of place. Anyone know what's wrong?!? Thanks <3