I got a bad sunburn with blisters, now I can't bend knee and foot is swollen; it's difficult to walk. Knee and thigh area looks bruised.

GET CHECKED OUT! This is concerning - you had second degree burns and now have complications of decreased range of motion, swelling and pain with weight bearing. While these may all just be results of the burn, it is also possible that there is a secondary infection, and if you ignore this at this point could turn into a disaster. Have it checked out today......