Hello @sarahshimek, Welcome to Mayo Clinic Connect. Thanks for sharing your experience and symptoms with Erythema Nodosum. You bring up an interesting question about the possibility of gluten being associated with the knot on your forehead. Here's some information I found that may explain it.
Skin and coeliac disease, a lot to think about: a case series: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5786986/
Excerpt – "Multiple erythematous violaceous nodules on the lower leg. … boy presented a 1-year history of an intensely pruritic skin rash located on the trunk, members and face. … Erythema nodosum in association with celiac disease."
Have you tried going gluten free for an extended time period as a test?