What Causes A Red Bruise Below The Elbow?

I have a red like area in the crook of my elbows (both). My doctor looked at it and did not know. It is not painful or swollen, it is not from an insect bite, nor it is a bruise. It is just a red area about the size of an orange. It seemed to start when I was wearing long sleeved shirts in the winter, but I am not doing that now and it is still happening. Thanks.