This sounds very much like lymphoma! Have it looked at immediately by an infectious disease doctor. Your knee and vein problem could be venous insufficiency--they often go together.
Have you seen a doctor? What was the diagnosis? What tests have you had? Do you have a fever?
'm sorry you're in pain; it sounds bad, but it's not specific enough for anybody here to help you. Assuming this did not come up overnight, you need a physical exam with a primary care physician to get you on the right track for diagnosis and treatment.