Skin lesion that reappears

[reeec199]

Hi, I have a mark on my back that is around 5mm and is purple with a lighter ring around it. I appears for 4-5 days and then goes down to just a brown fleck on my skin. It has reappeared at least 8 times over 6 months. I've seen 2 doctors as at first I thought it was a mole and they are completely stumped.
Does anyone have an idea what this could be?
Thanks