It seems like you have sprained a ligament in the knee although it's possible that you are manifesting with early signs of osteoarthritis if you are older than 50.
There are various ligaments of the knee that are very susceptible to sprain and tear due to excessive shearing force. They derive nutrition from the synovial fluids around the joint and hence do not heal very soon. Ligament sprain usually require conservative treatment whereas a tear will require a surgery.
I would recommend the following-
1)Consult your physician and obtain a X-ray of the knee in AP and lateral views. This will help to determine if there is any gross deformity or trauma like fracture. X ray doesn't help to diagnose ligament pathology.
2)Consult an orthopedic an and discuss the need for an MRI of the knee. MRI shows any pathology of the knee beautifully and helps determine if it’s a sprained ligament or a tear.
3)If it is confirmed it's a tear you will need a surgical consult and based on the age and the degree of mobility required, a surgery may be performed.
4)If it is a sprain, you will need complete bed rest for about a week and then you will have to restrict your physical activity. Ice packs and cold compress will reduce the inflammation.
5)Take Tab Tylenol 500 mg to reduce the pain and inflammation.