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 help for bleeding with fainting, confusion, severe weakness, chest pain, trouble breathing, or cold, clammy skin.
Also seek emergency care for severe or worsening pelvic pain when pregnancy is possible, particularly with shoulder pain, faintness, or one-sided pain.
And for anyone, someone who:
is bleeding heavily and it will not stop with firm pressure
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 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
Get same-day medical advice if you soak through a pad or tampon every hour for two hours, especially with dizziness or breathlessness.
Prompt advice is also needed for bleeding during a known or possible pregnancy, or bleeding with fever, severe pain, or foul-smelling discharge.
3
Does any of this apply?
Book an appointment
Book an appointment if bleeding:
Keeps returning or becomes heavier
Lasts longer than your usual period
Happens between periods or after sex
Disrupts sleep or daily activities
Causes possible symptoms of anaemia
Begins after menopause
The article also says
Light postmenopausal bleeding is not usually an emergency if you otherwise feel well, but it should be checked promptly.
Every sign above is quoted from the sofidex article on Abnormal uterine bleeding. If you are not sure, err upward — it is always reasonable to be checked.