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 care for a missed period or positive pregnancy test accompanied by severe or one-sided pelvic or abdominal pain, shoulder pain, heavy bleeding, fainting, marked weakness, or dizziness.
These can be symptoms of an ectopic pregnancy or significant bleeding.
A sudden severe headache, loss of vision, confusion, or new weakness also requires emergency assessment.
And for anyone, someone who:
Seek advice sooner if pregnancy is possible, symptoms are bothersome, or periods stopped after a uterine procedure.
Contact a clinician promptly for new nipple discharge, persistent headaches, vision changes, severe hot flashes at a young age, or rapid development of facial hair or a deeper voice.
Arrange a routine medical appointment if periods have stopped for three months without a known explanation.
Follow-up testing may be needed because hormone levels can change and one result does not always establish the cause.
The article also says
Keep a record of bleeding, pregnancy test results, medicines, weight or exercise changes, and related symptoms.
This information can make the evaluation more focused.
Every sign above is quoted from the sofidex article on Absent menstrual periods - secondary. If you are not sure, err upward — it is always reasonable to be checked.