I know that I can't take heavy blankets but I sometimes need them because I get so cold.
However, when I have to use them I wake up in allover body pain from the weight. I'm a side sleeper most of the time though so the blankets aren't resting on my stomach when I sleep. Are you a back sleeper?
Edit: I haven't noticed if it causes PEM though. It may but I'm usually focused on the pain instead.