knee injury.

Just looking for bit of advice.
had a blackout a few months ago(3 months approx). The day after it was very sore to walk on but the pain eased off after a week or so but whatever way i fell i have damaged my knee.

I can still play sport no problem . However feels weird to walk on.
If i sit down for a while or drive for a while my leg feels very stiff when i go to walk on it.
when i say stiff i mean doing a simple quad stretch is sore the first time then loosens out.
Another example is after throwing darts for a while the same thing happens.

The only stretch where i can feel any pain is when you are on your knees and try to sit on your heels.
I have went to the doctor and they cant find anything. Im not one for getting injured easily.
Just wondering has anyone ever experienced anything like this or any opinions.