What Does A Bruise On The Knee Indicate?

I have a large very dark purple bruise on my knee, 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 nothing was there no bug and no sign of any nearby. So about 5/10 minutes pass and I had to run inside because the stinging was still happening and I thought maybe the bug was trapped in my leggings (I know common thought, right? Lol) but nothing. Looked at my knee and it was red, but seemed to have a little knot type thing in it. Didn’t hurt just still kinda stung but not bad at all. There is not any bite mark and hasn’t been one. But now the center is light/not discolored and the surrounding is dark purple kindof oval shaped and it has only gotten worse since the day it happened. What could this be?