Belated happy birthday
As for your back pain and knee pain...I'm not a doctor, so take this with a grain of salt
I used to have bad knees (they used to crack a lot). But I started lifting weights, including strength training exercises for my legs and the muscle has seemed to alleviate the pressure on the actual knee joints. I used to get these aching pains in my knees from playing sports like basketball, but when I don't get those any more.
I've also read in Men's Health that working out your abdominal muscles helps to relieve stress on the back because the abs help keep you upright.
I know a lot of girls think lifting weights is weird but you can do it so you don't bulk up but just tone up.
Anyways...back to working out (guess that's why I suggested this!)
Now, if I was only a good dancer maybe I could feel how much pain you're in
-GL