Avulsion fractures of the patella are excedingly rare - and would usually require a high vélocity injury to come about. Have you had an xray diagnosis to confirm it? I've never heard of anyone feeling an extra piece of bone around the patella - whereabouts is it - above or below? How much pain are you in? After 5 days an avulsion fracture would still be seriously painful.
It's not impossible, but an acute quadriceps (above) or patellar (below) tendinitis would be much more common.
