I was recently diagnosed with osteoarthritis as well as slippage of my kneecap and your symptoms sound pretty similar especially since you danced for all those years. Have them looked at. I had X-rays done to my knees and did some physical therapy which helped a lot. I then decided to take up cycling to help strengthen the muscles that kept my kneecap in place. It's really helped me out a lot. Good luck and keep us posted!