Neverending Knee problems?

Ever since I was in 2nd grade I've had knee problems. There was no injury that anyone can recall back then. I just woke up one Monday and my left knee was extremely stiff. Over the course of the week it got so bad that by Friday I couldn't even put weight on that leg.

I'm nearly 30 now and have suffered with knee pain ever since. I've had 3 diagnoses. First, disease of the kneecap, 2. Water in the joint, 3. Slipping patella. I was told my kneecap could be slipping between my two legs bones.

The doctors are looking at xrays taken more than 10 years ago, I had an MRI done last year. Nothing shows up. Yet every now and then out of the blue it seems I get extreme knee pain.

I've done no more activity in the last few months than fast paced walking and light, short live jogging, (not more than a few feet) over the last several months. Yet this week I find my knee to be in excruciating pain.

There is a tiny dime size bruise near the kneecap but no other obvious signs of injury. I don't remember twisting it or anything. It just hurts like he11.

I've been battling this nearly all my life. There is a history of knee issues on both sides of my family. I'm just at a loss as to what this could be. No one can find anything wrong with them. There's something wrong or they wouldn't feel like this.

Any ideas anyone, help. Got any ideas I could share with my doc? I saw a sports medicine specialist as a teenager, and saw an orthopedic surgeon last year. Same result. Of course by the time I actually get to the appt. with those specialist the pain has cleared up.