The first thing that came to mind was erythema nodosa. This can be a stand alone diagnosis, or an accompaniment to several other conditions, many of which are autoimmune, although SjS hasn't been directly connected to my knowledge. Conditions that are associated with erythema nodosum include medications (sulfa-related drugs, birth control pills, and estrogens), strep throat, Cat scratch disease, fungal diseases, infectious mononucleosis, sarcoidosis, Behcet's disease, and inflammatory bowel diseases (Crohn's disease and ulcerative colitis). The last three are AI or AI related.
This isn't really rare or difficult to diagnose, so I doubt that your doctor would have missed it if is indeed EN.