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 if the leg or foot becomes cold, pale, blue, or numb after an injury.
Chest pain or sudden breathlessness with new leg swelling is also an emergency.
Seek same-day medical care if your knee becomes hot, red, very swollen, and painful, especially if you have a fever or feel generally unwell.
A joint infection needs prompt treatment.
Sudden severe swelling without a clear cause also needs urgent assessment.
Get urgent help after an injury if you cannot put weight on the leg, cannot move the knee, heard a crack, or the knee changed shape.
Rapid swelling soon after a twist or impact can suggest a significant internal injury.
Arrange follow-up sooner if the knee repeatedly locks or gives way, or if swelling does not settle.
And for anyone, someone who:
Book a routine appointment if pain lasts for several weeks, keeps returning, disturbs sleep, or limits work, exercise, or everyday activities.
Every sign above is quoted from the sofidex article on Knee pain. If you are not sure, err upward — it is always reasonable to be checked.