Naw, not normal, enfilade. You must have some sort of metabolic thing going on that the docs can't find. A similar thing happened to a friend of mine. The docs took muscle biopsies but still couldn't find the cause. It's gradually gotten better for her over the past couple of years.

Do not train harder or lift weights! My only suggestion I've made before, which is to take a total of about 60g of whey protein/day and see if that helps, while continuing to only ride slowly around the block. Just do it, and report back after a couple of weeks.