After treatment for thyroid cancer IBS getting debilitated after a BM

I've been through it all with orbital decompressions radiation chemo brain biopsies autoimmune system disease my biggest concern is why do I get so debilitated and sick after a bowel movement not to mention that I have fainted and falling off the throne several times throw up 15 years of studying I have tried to learn and eat and take vitamins to help my doctor is wonderful no one can answer this question so much appreciate if anyone else can tell me anything else I can do can someone please speak about this?