Brief Answer:
Due to osteoarthritis.

Detailed Answer:
Hello,
Thanks for posting your query.
The pain that you are having is likely to be due to osteoarthritic changes in the large weight bearing joints.
These are basically age related degenerative changes in the weight bearing joints like knee and is characterized by joint pain and stiffness. Upon movement the stiffness gradually relives. Sometimes application of hot pads can make the movement possible.
The treatment options include pain killers, hot fomentation, lubricating agents like glucosamine sulfate, chondroitin , dycerin, etc and some life style modifications and physiotherapy.
Right now you can start taking Tablet Tylenol ( Acetaminophen) Double Strength after breakfast and after dinner. It is suggested to add calcium supplements 500mg each twice daily. This will take care of your mild symptoms in 3-5 days.
You can start with walking and aerobic fitness exercises, quadriceps strengthening, patellar tapping, lateral heel wedges, etc- under the guidance of your physiotherapist.
A Big No to Stair case, squatting postures, any chair which has a sitting platform lower than your knees.
Knee brace can also be used for an earlier and prolonged relief.
In severe cases intra articular injections of steroids and hyaluronic acid may be needed after consultation with your orthopaedician.
I also encourage you to consult an orthopedician for physical examination. A few blood tests, X-rays of the knee, CT / MRI scans of the knee will help us diagnose the condition and help us in choosing the best approach of treatment.

I hope this answers your query.
In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.
Wishing you good health.
Regards.
Dr. Praveen Tayal.
For future query, you can directly approach me through my profile URL http://bit.ly/Dr-Praveen-Tayal

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