Looks like "port wine stains " to me.
Which is a type of birthmark.
My dad has those, and he finds they are more noticeable during certain times of the year as well...because of weather conditions I guess.
You would need to see a dermatologist in order to be positive.
Not sure where you live, but could it be some sort of bug bite?
My profile pic has the rash I'm talking about. They started as a solid red area then expand to a red ring with a clear spot in the middle.