Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Toxic megacolon is always a medical emergency.
Call emergency services or go to an emergency department now if you have severe bowel symptoms together with any of the following:
A rapidly swelling or very tender abdomen
Severe or worsening abdominal pain
Fever with a fast heartbeat
Heavy rectal bleeding
Fainting, confusion, marked weakness, or unusual sleepiness
Repeated vomiting or inability to keep fluids down
Sudden severe pain or a hard, rigid abdomen
If you have inflammatory bowel disease and develop a clear increase in diarrhoea or bleeding without the emergency signs above, contact your usual care team promptly.
Early review may help prevent a severe flare from progressing.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
The article also says
Do not drive yourself if you are faint, confused, severely weak, or in intense pain.
Do not take an antidiarrhoeal medicine to delay seeking care.
Every sign above is quoted from the sofidex article on Toxic megacolon. If you are not sure, err upward — it is always reasonable to be checked.