Yep, I had patella-femoral syndrome, which has since become floating knee caps - translation, my knee caps don't track as they should. Stairs were a big thing for me (grasping the railing sounds very familiar).
Um, you can do a quick eyeball check if this is something you have by standing up square with your knees over your toes and slowly bending your knees to about 90 degree. When your legs are bent (you may need a mirror) look to see if your knee caps are directly over your toes or not. If not - welcome to the club (lots of young women have it), believe it or not, you can survive this, and get lots of relief (little things like careful shoe shopping can make a big difference).
you might want to check out knee1.com for some good resources of info for knee problems. I know it's scary facing all this, but the more you know the less scary it becomes.