Pruni i'm still vague on it, but when i got these spots, the bloodtests showed increased levels of something - POTASSIUM! i remembered - and that usually indicates you've been vomiting, or eating loads of bananas, and i'd not been doing either, but the spots still appeared - under the skin, the sort that don't fade with the glass test.
Anyway, never got to the bottom of it, and still get little clusters of them sometimes, but they fade.