Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Emergency care is needed for widespread blistering or peeling, especially with fever, mouth or eye sores, facial swelling, trouble breathing, faintness, or symptoms beginning after a medicine.
Rapidly worsening pain, confusion, or severe illness also requires emergency assessment.
Seek prompt care for increasing pain, warmth, swelling, pus, spreading redness or discoloration, an unpleasant smell, red streaks, fever, or feeling generally unwell.
Get urgent medical help for eye pain, marked light sensitivity, sudden vision changes, or an erosion involving the eye.
Arrange routine medical advice if an erosion has no clear cause, keeps returning, is not starting to heal, or is growing.
Assessment is also appropriate for mouth or genital erosions, multiple blisters, or symptoms that began after a new medicine.
The article also says
People with diabetes, poor circulation, reduced sensation, or a weakened immune system should seek advice early, especially for an erosion on the foot or lower leg.
Every sign above is quoted from the sofidex article on Erosion. If you are not sure, err upward — it is always reasonable to be checked.