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
Get emergency help if the foot becomes suddenly cold, pale, blue, numb, or severely painful.
These signs may indicate reduced blood flow.
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
Seek prompt medical advice after an injury if you cannot put weight on the foot, there is marked swelling, or the foot looks deformed.
Urgent assessment is also needed for a rapidly spreading red, hot swelling with fever, or for an open wound with exposed tissue.
3
Does any of this apply?
Book an appointment
Arrange a routine appointment if:
Foot or ankle pain lasts, returns often, or limits normal activity
One foot becomes flatter or changes shape
There is persistent swelling along the inner ankle
The foot is stiff or you cannot rise onto your toes
A child has pain, stiffness, walking problems, or unequal feet
Shoe changes and simple self-care do not improve symptoms
The article also says
If you have diabetes, poor circulation, or reduced foot sensation, seek early advice for any new blister, cut, ulcer, redness, or swelling rather than waiting for it to become painful.
Every sign above is quoted from the sofidex article on Pes planus. If you are not sure, err upward — it is always reasonable to be checked.