I have severe discomfort in my joints. especially in my knee and ankle. my vitamin d is very low and is this the reason?

Probably not: Vitamin d is associated with osteomalacia or softening of the bones and bone pains. Joint discomfort is more commonly seen in arthritis. At age 30 with generalized joint pain you should be evaluated by a rheumatologist to see if you have an underlying condition that can be treated.

I don't think so: But, if you know it is low why aren't you supplementing to raise the levels to normal? Are you flatfooted or pronate? If so try arch supports. If not, see an orthopedist or rheumatologist for follow up of the joint pain.