Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help if bleeding is heavy and you feel faint, collapse, become confused, or have chest pain or trouble breathing.
And for anyone, someone who:
If pregnancy is possible, get urgent help for bleeding with severe or one-sided abdominal pain, shoulder-tip pain, marked dizziness, or fainting.
These can occur with an ectopic pregnancy, when a pregnancy develops outside the uterus.
Heavy bleeding or severe pain during pregnancy also requires urgent assessment.
Contact a healthcare service the same day if you are pregnant and have vaginal bleeding, even if it is light.
You should also seek prompt advice for bleeding with fever, worsening pelvic pain, or foul-smelling discharge.
Book a routine appointment if:
The article also says
Postmenopausal bleeding should be checked even if it happens once or is only a small amount.
It is not usually an emergency unless it is heavy or accompanied by other urgent symptoms.
Every sign above is quoted from the sofidex article on Vaginal bleeding. If you are not sure, err upward — it is always reasonable to be checked.