Thank you for consulting with Doctorspring. I understand that you've been investigated in detail for your symptoms and your sigmoidoscopy and stool tests have turned out negative. Your symptoms could suggest a myriad of diagnosis including parasitic infection, inflammatory bowel disease, lactose intolerance, malabsorption and finally irritable bowel disease. Most of the diagnosis can be ruled out from the fact that your stool tests have turned out negative. But irritable bowel disease comes into the picture since all the diagnoses have been ruled out.
Irritable bowel disease is known to also have some psychological triggers like depression and anxiety. It mainly presents with abdominal pain which is relieved by defecation. IBS can only be diagnosed if all other diagnoses with similar symptoms are ruled out. Also, IBS has a female predominance. IBS may be diarrhea predominant or constipation predominant. There are several medications depending on your type of IBS. You should talk to your GP regarding IBS and possible medication to relive your symptoms.
About endometriosis, it can be safely ruled out since you have had a hysterectomy. It does not present with only gastrointestinal symptoms. If endometriosis involves the colon and rectum, it could present with pain during defecation and blood in your feces. You have not mentioned the reason for your hysterectomy with may throw further light on proper diagnoses. I hope the information was useful to you.