It's most probably an allergic reaction, I visited a dermatologist last week and it turned out a massive reason for my EC was lip eczema, and the steroid he prescribed me is helping massively, I've tried steroids before and they were no help but this one is working, so you need to have the right steroid prescribed.
Bear in mind I've had this for 1 year and I'm only now finding out that my EC was cause by eczema.