For almost 2 weeks my stool looks like coffee grounds, sometimes dark brown or black. What does this mean? Headache, neck pain, lost 90 lbs in 6mo unintent

Black stool. It sounds like you might be experiencing bleeding from the GI tract. Please seek help immediately.
Bleeding. You may be having bleeding somewhere in your gut. This plus the unintended weight loss doesn't sound good. See your doctor.