What Causes Recurring Bouts Of Nausea And Weakness?

Hi. I am a 61 y.o. female. Every month or so I develope the following symptoms. Sudden onset of nausea and weakness followed by a large soft bowel movement. Some sweating generalized weakness. The nausea sometimes progresses to vomiting. After a nap sometimes with chills I feel better. Not great, but like I am gonna live. I have not been able to narrow down any precipitating events. Have not been constipated. or ill in any way. My own doctor is stumped. I do have hypothyroidism and my levels have been checked and appear to be wnl. Any ideas?