Welcome to Just Answer.
Can you tell me if your knee if swollen?
Does it feel hot or look red?
Thank you Martha.
To help alleviate the pain.
Try wrapping it with an Ace bandage.
Not too tight but just snug enough.
Remove the bandage when you go to bed.
Keep the leg elevated as much as possible.
Try an ice pack for 20mins at a time.
Alternate that with a heating pad.
That should help it start to feel better.
My other thought though since you don't remember injurying it could be a blood clot.
Signs of a blood clot would be:
If that develops get to the ER ASAP.
Try to stay off of it as much as possible until you can see your doctor.