Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for severe difficulty breathing, fainting, uncontrolled bleeding, sudden severe chest pain, or signs of stroke such as facial drooping, arm weakness, or trouble speaking.
And for anyone, someone who:
After discharge, contact the transplant team promptly for fever or chills, new cough, vomiting or diarrhoea, worsening breathlessness, swelling, rapid weight gain, palpitations, reduced urine, or redness, drainage, or opening of the incision.
Also call if you miss or cannot keep down an anti-rejection medicine.
And for anyone, someone who:
Attend routine appointments even when you feel well.
Use these visits to discuss medication effects, sleep, mood, rehabilitation, vaccination, work, travel, and sexual activity.
Every sign above is quoted from the sofidex article on Heart transplant. If you are not sure, err upward — it is always reasonable to be checked.