Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
If the exposure happened within the past 72 hours, seek urgent medical care immediately to ask about PEP.
Seek urgent or emergency care for severe breathing difficulty, chest pain, confusion, a seizure, fainting, a severe headache with neck stiffness, sudden weakness, or inability to stay awake.
These symptoms have many possible causes, but they need rapid assessment.
Arrange prompt testing if you may have been exposed to HIV, even if you feel well.
Arrange confirmatory testing promptly.
Until the result is confirmed, avoid sharing injection equipment and use measures that reduce sexual transmission.
The article also says
Contact your usual HIV service soon if you:
Have missed several doses or cannot access your medicine
Develop new or persistent symptoms
Have troublesome treatment side effects
Receive a detectable viral load after previously being undetectable
Are pregnant, planning pregnancy, or breastfeeding
A positive home HIV test is not usually an emergency.
Every sign above is quoted from the sofidex article on HIV & AIDS. If you are not sure, err upward — it is always reasonable to be checked.