I was just on vacation in North Carolina and when I got back I noticed two large, itchy, dark red/purple spots on the back of my knee. The spots are blotchy and have darker purple centers. They don't fade with pressure. Ones about the size of a quarter and the others bigger. It could be poison ivy or mosquito bites, but it didn't look like the pictures I've seen. I don't think i'm allergic to anything. Any idea on what it could be? Should/when should I go to a doctor?