You could have tendonitis???
It might even be a stress injury to the ligaments of your knee???
Any other symptoms you could add:
less range of motion at the knee...
increase pain in the morning and evening? (pain subsides during the day as you warm up?)
just as a precaution I would stop playing tennis
It's not in the tendons. It is in the muscles around the knee. Also, my right calf gets sore and very weak but not all the time. I also can get severe pain in my left hip when sleeping on it at night. I have tendinitis in my wrist and this is not the same. This is stiffness and sever ache through the whole knee area in both knees.
Pain is also bad in the morning when i wake up since the knees are very stiff. I first thought arthritis but this is definitely more than joint pain.
It may be a torn meniscus. I have joint stiffness in my knee as well as pain along the inner bone and a little swelling on both sides. My PM doctor thinks it's a torn meniscus because my xray showed some inflammation.
I also have the other symptoms you have. The muscle behind my knee aches and the pain is worse when I'm not moving. I thought mine was arthritis as well but the xray proved otherwise. I just had an MRI done so I'm waiting to see if I need surgery.
I have a compression brace that I wear on bad days. Which is almost everyday since my office is up a flight of stairs, which makes it hurt so bad.