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
Seek emergency care for:
Severe or rapidly worsening abdominal pain
A hard, swollen, or extremely tender abdomen
Fainting, confusion, severe weakness, or difficulty breathing
Repeated vomiting or inability to keep fluids down
Heavy or continuing rectal bleeding
Severe pain with fever and shaking chills
Inability to pass stool or gas with increasing swelling or vomiting
These symptoms can indicate a perforation, obstruction, major bleeding, dehydration, or widespread infection.
Do not drive yourself if you feel faint, confused, or seriously unwell.
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
2
Is any of this happening?
Be seen today
Arrange prompt medical assessment for new, persistent lower abdominal pain, especially when it occurs with fever, nausea, bowel changes, or marked tenderness.
Contact a healthcare professional again if diagnosed symptoms are not beginning to improve, return after improving, or make it difficult to drink.
A lower threshold for urgent assessment is appropriate if you take immune-suppressing medicine, are receiving cancer treatment, or have had an organ transplant.
The article also says
People with a weakened immune system may have less obvious fever or inflammation despite serious illness.
Every sign above is quoted from the sofidex article on Diverticulitis. If you are not sure, err upward — it is always reasonable to be checked.