10 POINTS How can I stop my joints from feeling so stiff?

I am only 19 years old, and I do not have athritis or anything. I drink loads of milk but have done karate for a number of years. The joints in my big toe, knees, right hip and fingers often feel stiff, tight and occasionally sore until I crack them. Then there is momentary relief. This happens most when I'm in bed or sitting around. Does anyone know of ANY tips or solutions to help relieve this stiffness? Any help would be much appreciated because every time I tell my mum, she just tells me I've formed a habit when in actual fact, my bones actually do feel stiff. Please help!!!