What Causes Erythema Nodosum On Lower Legs?

I'm pretty sure I have a case of erythema nodosum that started showing itself on my lower legs 11/2 weeks ago. Coincidentally, I experienced a day of stomach cramps and then bloody diarrhea 1 week ago. Perhaps coincidentally, again, I had sinus surgery 2 weeks ago, for which a 10-day course of Keflex. My ENT wanted me to do another round of the Keflex b/c of continued yellow drainage. I started the second round 24 hours before the diarrhea began. A lot of coincidences, or can this all be related? The diarrhea is gone and my gut seems fin (I have discontinued the antibiotic). My new worry is where the EN has come from. How worried should I be? Buzz words like ulcerative colitis and sarcoidosis and lymphoma are stuck in my head.