Knee issues at night.. Help

Okay, I will try to explain this as best as I can..

For the past few weeks.. as soon as I lay down it feels like my knee is locked straight and it begins to hurt badly, and it takes all my energy and force to bend it.  I have tried pillows under it or between  my legs... but as soon as I find a comfortable position and fall asleep I wake up to intense pain again and it takes me a good 10 mins to get my leg to bend... the pain is the worst feeling ever.. Once I wake up with it and bend it, I will be up for hours due to it aching. I have no injuries to this leg at all.  what could be causing this???  most nights I will have to fall asleep cross legged in bed just so I dont have that pain.. hoping this makes sense .. sorry.

[Dr. Kokil Mathur]

Hi!
When you lie down the muscles around the joint relax. This causes rubbing of the bones around the joint. There are two conditions which cause pain while at rest or sleeping. One is arthritis, preferably osteoarthritis and the other is tendonitis. This is typically seen in osteoarthritis in initial stages. While standing and walking the muscles keep the knee joint stable. On lying down and especially if the cartilage is wearing off the bones of thigh and calf can rub against each other and cause pain. Being overweight also adds to the strain on knee joint and hastens the wear and tear of cartilage. Since both the knees are affected rheumatoid arthritis too is a possibility. Another possibility is ankylosing spondylitis. Though this primarily affects the spine, it can affect the knee joint too. This disease causes pain that is worse on lying down and improves on standing. It is beyond the scope of net to give a confirmed diagnosis. Other symptoms, investigative procedures and clinical examination are a must for confirmed diagnosis. Please consult your PCP for primary examination followed by proper referral.
Take care!