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 difficulty breathing, swelling of the face or throat, widespread hives, sudden severe dizziness, or collapse.
A seizure, inability to wake normally, or another major change in consciousness also requires emergency care.
Seek prompt medical advice for a high or persistent fever, marked limb swelling, unusual weakness, or symptoms that are worsening.
The article also says
Contact your healthcare team if soreness, redness, swelling, fever, or other expected effects do not improve after a few days.
Also call if a reaction concerns you, interferes with normal activities, or you need help planning the next dose.
Every sign above is quoted from the sofidex article on Diphtheria toxoid vaccine, inactivated. If you are not sure, err upward — it is always reasonable to be checked.