clicking and cracking joints?!

hey everyone :)

I've had clicking and cracking joints since i can remember particularly in my wrists and knees but it can also happen in my hands and fingers, back and shoulders and ankles....
My knees make a horrible crunching sound every time i stand up from crouching down and my wrists crunch when i do circular motions with them...
None of these particularly hurt but they can be achy if they feel like they need cracking and my knees especially feel strained when they are crunching... not very nice.
I also get an achy lower back quite often...nothing unbearable but its uncomfortable and makes sleeping and standing still difficult.

Does anyone know what could cause this? Am I lacking a certain nutrient or vitamin? Is it totally normal?

I'm pretty average... 5ft 8, 23, 158lbs, female. I have a fairly active job which involves being on my feet most of the working day, walking around, going up and down ladders, moving things and lifting things.... i do 4-5 hour long workouts a week with cardio and weight lifting... and walk home everyday which takes 20 mins. weekends I'm nowhere near as active.

any help would be appreciated! thanks in advance :)

xx