The most likely cause is a joint mouse, that is a loose body, perhaps a piece of meniscus or a bit of articular cartilage floating around in the joint. When weight bearing the mouse is most likely pushed away into the folds of the joint and therefore no pain is felt. When in bed the joint is loose and the mouse gets trapped between the joint surfaces. Usually if you move the knee around it will disappear. I have them in both ankles and usually get them on walking but only rarely in bed. Often the xay looks OK as the mouse may be translucent. If it is too troublesome your doc may recommend having it arthroscoped if of course that is the cause.
If I didn't know any better, I'd swear I'd written your post myself. I am suffering with the same type of knee pain for about 2 weeks now- wakes me up in the night and I have trouble getting comfortable again. The pain starts in the back of my knee and shoots down my leg and into the top of my foot. One morning it was so painful when I got up, I was in tears by the time I got to the kitchen. Once I am up and moving around the pain almost completely goes away.
I went to my primary care doc last Thursday, and she didn't think much of it, it had only been a week, etc. She did do an Xray, and I got the results today - they say I have a loose body, and I'm scheduled for an MRI this Thursday.
I am 53 and just started getting really athletic the last couple years. I did my first full triathlon in August, so I sure don't want some major set back. I have a plan for the winter to get ready for another tri or two next year!
Blitz Mom, Glad you are having something done and perhaps you will have it scoped and then it will be gone. My daughter is a about a year younger than you and she will be doing a marathon this coming weekend, it's great to be physically active, gives you a good outlook on life.
James079 - good luck to your daughter this weekend!
I have to qualify myself a little here - I didn't do a full blown distance Triathlon - no Iron Woman here! But I did do what is called a Sprint Tri - 1/4 mile swim, 16 mile bike, and 5K. I am not a swimmer, and I didn't decide to do this one much ahead of time (6 weeks?), so I swam from one kayak to the next. I love the bike part, and that is the main reason I signed up, so no problem on the bike. For the 5K I planned to walk most of it. I don't have great knees to begin with, and I figured I'd be exhausted, but I do walk pretty fast. I ended up jogging about 1/2 of it (adrenalin rush). I had also done the 5K leg of a tri in August 2010, and finished this 5k in nearly the exact time as last year after swimming and biking! I was pretty thrilled to finish in under my goal time which was 2.5 hours.