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I'm going to do some research for you and give you some suggestions, but could you answer a few questions for me first?
Do the areas change location, or do the same areas get red and raised each time? Is the rash only on areas exposed to the sun, or are they other places, too? Do they hurt when you touch or push on them? Do they itch? Do you take any medications? Do you have any medical problems? How old are you? Is there a ring around the rash? Is the rash made up of separate raised red areas, or does it all run together?
Thanks for your help. I look forward to hearing back from you.
OK, what this sounds like to me is a rash called pityriasis rosea. It occurs on the body but generally not on the face, is caused by a viruses, tends to last many weeks and up to 3 months, and tends to occur in the spring of the year. It either is mildly itchy or doesn't itch at all. There isn't any treatment. IT doesn't seem to be particularly contagious, as I've often seen one person in a family have it and no one else come down with it. There isn't any treatment and it eventually goes away on its own. The brighter redness with exercise or sun doesn't mean a lot, and isn't anything to worry about as many rashes react that way to heat. Some people notice a little scaling of the rash areas, but that is usually pretty mild. Usually there is one larger spot, often on the upper trunk or upper arm or back; this will be an inch or two in diameter and is called a herald spot. If you have that, then the diagnosis is pretty certain...
If you don't have a herald spot, then you start thinking about things like nummular eczema, which is usually itchy with slightly firm lesions, that can be scattered over your body. These usually have a roughened surface. It doesn't sound like ringworm, as usually you don't have more than one or two spots. It doesn't sound like hives as the lesions don't completely go away. It could be a reaction to drugs, but you aren't taking any; and anyway, even if you did take something three weeks ago, that would be long gone from your system by now. A sun reaction would only be on sun exposed skin, so that doesnt' seem likely. Bug bites could look like this, but they,too, would be gone by 3 weeks, and to have that many of them you would have to be aware of getting bit.
So, I'm back to the pityriasis rosea. If you google it there is a site that has tons of pictures so you can compare. Hope this helps. If this fits your description then you probably don't need the dermatologist, as it is a benign condition that goes away on its own. If it changes or you develop symptoms or it spreads to your face, that may be a different matter.