I was outside and felt burning on the back of my knee. i felt like i was getting sun burned. now 2 days later i have a really bad dark bruise.

Get checked please: May be sunburn although a dark bruise doesnt sound characteristic. Im wondering whether you have had something like a bleed into a ruptured bakers cyst or some other vein. Theres an outside chance of it being more serious such as deep vein thrombosis. Without examining is a difficult to tell. Please see a doctor urgently for examination as if not clear you may also need to be referred for tests

Bite : You may have been bitten by an insect and the bruise is what is left. Leave it alone and it will gradually disappear