Women's Health Community

Dear An: For the sake of other readers, here are a couple of links explaining erythema nodosum (EN):

http://www.nlm.nih.gov/medlineplus/ency/article/000881.htm

http://www.webmd.com/skin-problems-and-treatments/erythema-nodosum

As you can note, birth control pills are mentioned as possible triggers in both these articles. In one the synthetic progesterone ("progestin") is also singled out while in the other, estrogen is singled out. So it MIGHT be your birth control pills (especially if there was a recent dose increase).

Here is one of 97 citations at the National Library of Medicine site on autoimmune EN:

Ann Dermatol. 2011 Aug;23(3):362-4. doi: 10.5021/ad.2011.23.3.362. Epub 2011 Aug 6.
A Case of Assisted Reproductive Therapy-induced Erythema Nodosum.
Jeon HC, Choi M, Paik SH, Na SJ, Lee JH, Cho S.
Source

Department of Dermatology, Seoul National University Boramae Hospital, Seoul, Korea.
Abstract

Erythema nodosum is a common variant of panniculitis. It is characterized by tender erythematous nodule and plaque on the anterior aspect of the leg. The etiology is not fully understood. It may be associated with a variety of disorders, including infection, medication, autoimmune disorders, pregnancy, and malignancy. A 33-year-old Korean woman presented with 1 week history of painful erythematous plaques on both knees. She was 7 weeks pregnant with assisted reproductive therapy, and had been maintained on daily intramuscular progesterone injection for 4 weeks. Histological examination of the lesions revealed septal panniculitis without vasculitis. Two days after discontinuing progesterone injection, the symptoms and lesions started to resolve. Herein we present a case of erythema nodosum caused by progesterone injection for endometrial preparation.

An_249423, this is the most black and white answer I could finds after doing multiple searches.

Yours,
Jane