Is it possible to get marks on your skin if you dont exercise for a long time?

I'm 16 and i have noticed red wavy lines appearing on my knees and thighs, they've been there a few weeks and keep noticing new ones appearing. Due to an illness i haven't been able to do very much exercise for a year but i'm not over weight. They do look a bit like stretch marks but i dont think they are because they came so suddenly and the skin hasn't been stretched, what could they be and are they related to not doing any exercise?