Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency eye care now for sudden or worsening vision loss, significant eye pain, or increasing redness—especially after eye surgery, an eye injection, or a penetrating injury.
Contact the emergency number provided by your eye clinic or go to an emergency department with ophthalmology support.
Call emergency services if eye symptoms occur with severe illness, confusion, breathing difficulty, fainting, extreme shivering, or rapidly worsening weakness, as these may be signs of sepsis.
Also seek urgent assessment for new floaters, marked light sensitivity, eyelid swelling, discharge, or a cloudy appearance inside the eye after a procedure.
Do not wait for a routine appointment or use leftover eye drops.
Contact the treating service sooner if pain, redness, discharge, or vision worsens between appointments.
The article also says
Once treatment has started, keep every scheduled review.
Every sign above is quoted from the sofidex article on Endophthalmitis. If you are not sure, err upward — it is always reasonable to be checked.