For me PEM is mainly inflammation, headaches and neck and stomach tightening for hours after a physical stressor, it almost always Involves heat, I can't get hot without huge brain fog afterwards eg 2 hrs of helping a friend do up some nuts and bolts on the sun, a bit of thinking and a tiny bit of stress, on the way home I forgot how to drive through simple traffic lights. I saw the colour of them but somehow I wasn't sure what to do. If I do exercise I boil on the inside (unless I take an anti inflammatory or panadol) heat killed me before I had cfs/me and often resulted in what seemed like heat stroke. My stomach would go freezing cold too. I frequently have a red swollen uvula and sore throat that looks a lot like mono, yet its gone 2 days later. Fatigue is there, with headaches and brain fog, a head feels like its full of fluid and a crappy mood that changes all over the place and I don't sleep well after exercise either. I used to get tired arms hanging out washing, just elevating them with a tea towel was enough to get a huge lactic acid 'burn', 5 years later it's not so bad