Are you positive that your bowel issues are IBS? The reason I ask is that erythema nodosum and peripheral neuropathy both occur in Crohn's Disease. Crohn's is autoimmune and has many extraintestinal manifestations. Many people with CD get uveitis/iritis. I have peripheral neuropathy from CD. I have also had erythema nodosum and iritis several times.

Perhaps the GI could order a Prometheus Serology 7 test if your insurance will cover it. It is somewhat pricey if you have to pay for it on your own.

BTW, have you had a colonoscopy to rule out inflammatory bowel disease? A diagnosis of IBS is one of ruling out all other possibilities, not just relying on symtpoms. If your IBS diagnosis was made without doing a colonoscopy, then that diagnosis is suspect in my book.