I have a large very dark ring/oval purple bruise on my knee with a light center/not discolored, but have had no trauma to the knee itself. I thought I may have gotten stung by something because it all started when I walked outside for less than a minute and felt a slight stinging sensation - my reaction was to dust off my knee, but no bug. Still no bite or sting mark. Bruise has only gotten extremely worse. What could this be? Day before a huge receiver fell on my back and didn’t even bruise..