What Causes Severe Diarrhea With Stool Color Changing From Yellow To Brown?

I am a 40 year old female. for the last 3 weeks I have had severe diariah. going at least 4-5 times a day. at first it was mucasy, yellowish brown color. It comes out just like I am urinating. but today the color has changed to a dark dark brown almost black color. normal bowel movements are usually normal sometimes hard to go. when that happens I take a mild stool softner and Im back to normal. but for the past 3 weeks this has been going on and now with the color change I am concerned. I have a family history of cancer, breast cancer, lung cancer, heart disease. my appetite has not changed but I do feel allot more tired than usual. I have occasionally felt nausiated but it passes. any suggestions on what this might be or what I should do?