My 'crashes' follow more cognitive or physical exertion than I should be doing but I don't know it until the days after the exertion. My headache varies in intensity-at it's worst, light/sound/smells are intolerable. My arms and legs feel like painful, heavy cement blocks that that require Herculean effort to move. I am lightheaded, nauseated, lack appetite. The incapacitating profound exhaustion makes me a lifeless glob of flesh. I am unable to summon the energy to get out of bed or off the couch. I pray I can fall asleep so I can stop feeling the pain. I slept 26 of 36 hours the last episode. Lots of water helps even though it means getting up to fill a glass, forcing myself to drink and then getting up again for the bathroom. Electrolyte replacement, Gatorade/NUUN/salt, helps as well. Excedrin Migraine is moderately helpful.
I was a marathon running, weight lifting, ER nurse before COVID in November 2020; now I keep an hourly log of what I am doing, identify if my activity is cognitive (daily planning, appointments, finances, grocery list, highway driving, insurance forms, support group, reading the news) or physical (walking, PT exercises, yoga, errands, laundry, washing & drying my hair, picking up the house, cooking). I tally up how much time I have used. I am done for the day when I have used up the 2hr limit for each. Sometimes that is noon, sometimes 4:00. It took ~4months to really pay attention to this before zeroing in on what my time limits are; a lot of trial and error, usually feeling good & thinking I can do everything, crash, repeat. Read up on "Post Exertional Malaise"-it fits me exactly.