I've been living with migraines for 40ish years. For migraines in game, I usually treat them as (insert triggers here) can induce a migraine lasting 1d10 (mild-moderate) 1d100 (moderate-severe) dice rolled hours. All actions are at -2CS for 1d10 to 2d10 hours during the worst pain, and -1CS for another 1d10 to 2d10 hours during the worst part of the migraine hangover (pro and postdrome effects). Migraines can be mitigated by medicine such as Triptan class Rx drugs or preventatives like Emgality or Ajovy etc (anti-CGRP drugs). Migraineurs must make a Psyche FEAT to focus and act during a migraine. An Endurance FEAT is required to work through the physical Pro- and Post-drome "migraine hangover. Taking the properly Rx'd medicines can shorten the length of the migraine from 1d100 hours to 1d10 hours or less and reduce the negative penalties for acting.
Some of the side effects of migraine can include Aura (-1cs to visual perception rolls due to flashing, clashing contrasting visual disturbances--google Migraine Aura for examples), heightened sensitivity to light, smell, sound (+1 to +3CS effects from such phenomenon), nausea, allodynia (sensitivity to touch, where even the gentlest, most casual touches like a cat brushing by ones skin, or a carer lightly wiping one's face/limbs with a cool/damp cloth can be very painful), dizziness, transitory aphasia (one can't use one's words, because they come out 'wrinkled'), synesthesia (where the brain doesn't process sensory input correctly---it's like smelling texture, tasting sound and hearing colours, or in other odd combinations to simplifiy it). There are others, but not every migraineur gets them all in the same intensity at the same time. They may have some side effects and symptoms that are more intense for them.
There's a lotta truth in wanting to hid in a cool, dark, quiet room when one's got a migraine going hellbore in your head. The pain is usually centrd in one place or one side and it's oft described accuraly as a pounding or pulsing or stabbing pain.
Migraines are worse when one wakes up with them (it's usually well-past the point where one's abortive meds like the Triptan-class Rx drugs can stop it. This notion could also work for cluster headaches and hemi-plegic migraines (Look those up on WebMD or Mayo / Cleveland Clinics websites for more details.)
The worse kinda of head pain comes from Tri-geminal Neuralgia, which effects the Tri-geminal Nerve. These are often called "suicide headaches" due to the horrible, unrelenting pain. (More info can be found on them at WebMD or Mayo / Cleveland Clinics websites).
I have allergies, but I'm not sure how to write them up. I'll have a think on that....