Black Spots In Stool

I have had dirrhea for many weeks now. Sometimes all water and color, sometimes there is actual stool in them. My question is, recently I've been noticing black spots in the small stools that I pass. I'm not on any special diet, or new meds. I have meds that I take regularly, but have never had a problem with them. I do get discomfort in my lower abdomen, and somtimes near my diaphram. I have lost a significant amount of weight recently, I get dizzy, light headed, my heart will start to hammer for no reason, and my muscles somtimes get weak. Could this all be linked? What could cause all of this? Thank you for your time. A.S.