I'm 24, and yet, I'm starting to get it where my joints hurt after exercise, not muscle.

I work on a till and have been for about a year where I work, so when I bend my fingers in to my palms, they ache.

I also play on dance machines a lot, and I did on Wednesday for about 4 or so hours with a mate of mine.

Not had it like this before, but I can't bend down very well at all. If I do, it really hurts getting up, at the knees, and behind the knee. It doesn't feel like muscle pain.
I know it is quite intense for your legs, as you are hitting something rather hard constantly, but I never thought I would get this.

I've always had it though where if I kneel down, my knees click, like if you are stretching. Pretty much almost every time too, and have been for as long as I can remember now. If I kneel a lot, like I need to at work, I can feel my knees starting to ache from doing it.

What does this mean?