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 immediately for difficulty breathing, swelling of the face or throat, collapse, confusion, or a rapidly spreading rash accompanied by breathing or circulation symptoms.
Seek urgent medical care for chest pain, shortness of breath, a fast or pounding heartbeat, or unexplained fainting.
These symptoms require assessment even if they are unrelated to vaccination.
For non-urgent questions about your record or another dose, contact the vaccination service or your usual healthcare team.
The article also says
Mild arm pain, tiredness, headache, or fever during the first few days can usually be managed at home.
Contact your healthcare team if symptoms are not improving, interfere with daily activities, or if redness or tenderness worsens after the first day.
Every sign above is quoted from the sofidex article on COVID-19 vaccine - what you need to know. If you are not sure, err upward — it is always reasonable to be checked.