Its been going on for almost a month now that i wake up some mornings and have bitter saliva in my mouth which bothers me so much, when i spit it out it is dark drown. what is the cause?

These sour eructions are one of the symptoms of GERD when acidic juices travel up the esophagus into the back of your throat. Cut out caffeine, alcohol, nicotine,aspirin, anti-inflammatory meds like ibuprofen or naproxen, spicy&oily foods and take a medication like famotidine or omeprazole for a week or so. If not improved, consult a gastroenterologist for appropriate testing(gastroscopy, h pylori