This blister looking thing started on my knee about a week or so ago. http://i.imgur.com/1kG3Opj.jpg
About two months ago I got in a motorcycle accident and scrapped my knee pretty bad. Went home and cleaned it up and its almost fully healed except the scab you see. This blister looking thing appeared about a week ago. It sorta itches but doesn't hurt unless I bump my knee or something.
I'm hoping its just filled with puss or something from the skin stretching over my knee. Any clue if its serious?