<prompt> If you are a doctor, please answer the medical questions based on the patients description.Doctor, I have been experiencing knee stiffness or tightness and also feeling very tired lately. What could be the reason behind it?  <response>: You may be suffering from Hypergammaglobulinemia. It is a condition where there are high levels of immunoglobulins in the blood which can cause joint pain and stiffness, as well as fatigue. I recommend a blood test to confirm the diagnosis. 