What Causes Bloody Stool?

Hi, I have had bloody stools now for 3 weeks. It isn t everyday but episodes of a couple days stringed together or just one day out of 3... Today, I started to feel nausea after I had passed bloody stools, and this time there was a lot of blood. Every other time there was a significant amount, something to be scared about. I went to the hospital and they said it was a hemroid from being constipated the last 3 weeks, but I alos gained 10 pounds suddenly... I m having weird headaches and cramping in my neck area and my left side feels funny and sometimes tingly. don t know if thats from this or just sleeping wrong. When I was constipated and had bloody stools, there was a pressure in my lower left side with slight pain if I laid on that side. That pain is now gone but I still feel bloated, gassy at times, and bloody stools won t go away. And the blood is not bright red, rather regullar to dark red, but it has been bright red at other times as well. After bloody stools, I feel an odd warmth in my stomach and nausea, and get gassy.