What Causes Red Discoloration Around Elbow That Turned To Purple Bruise?

I woke up with a circle shaped red discoloration around one elbow, definitely not an injury, appeared red in the am and now, second day is deep purple and looks like a bruise. Muscles or joint seem weak and sore. Two docs have been stumped and took blood, cbc came back no infection and other doc did several other blood tests and said I don t know and take some ibuprofen. It does not hurt to touch or bend and there is no skin problem like a sore or bite. I am worried that the doc said wait until Friday when blood comes back, any ideas?