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 help for swelling with severe breathing difficulty, chest pain, coughing blood, fainting, or sudden confusion.
Seek urgent same-day medical care if the affected area suddenly becomes red, hot, painful, or much more swollen.
Fever, chills, or feeling unwell may indicate cellulitis.
Sudden one-sided leg swelling with significant pain may be caused by a blood clot and also needs urgent assessment.
And for anyone, someone who:
Arrange a routine medical appointment for new or gradually increasing swelling, especially if it affects one limb or follows surgery, radiation, infection, or injury.
Also seek review for persistent heaviness, tightness, skin thickening, or repeated infections.
The article also says
Ask for timely advice if a compression garment causes pain, numbness, blue or pale fingers or toes, broken skin, or rapidly increasing swelling.
Remove a garment that is clearly cutting off circulation while seeking advice.
Every sign above is quoted from the sofidex article on Lymphedema. If you are not sure, err upward — it is always reasonable to be checked.