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
Call emergency services for:
New or severe chest pain, pressure, or tightness
Severe difficulty breathing or breathlessness at rest
Fainting, collapse, confusion, or difficulty staying awake
A rapid or irregular heartbeat with chest pain, weakness, or light-headedness
Blue or grey lips, or cold and clammy skin
These symptoms can occur with myocarditis, a heart attack, a dangerous rhythm, or another serious condition.
Do not drive yourself if you feel faint or severely unwell.
These changes need review but are not always an emergency.
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 if you have ongoing chest discomfort, unexplained breathlessness, repeated palpitations, or a marked loss of exercise capacity, particularly during or after an infection.
The article also says
If you have already been diagnosed, contact your care team about increasing ankle swelling, rapid weight gain over several days, worsening fatigue, new palpitations, or breathlessness that is becoming more noticeable.
Every sign above is quoted from the sofidex article on Myocarditis. If you are not sure, err upward — it is always reasonable to be checked.