Thank you for the query.
Dark stool can be caused by bleeding from peptic ulcer
. Especially if you also suffer from upper middle abdominal pain
after a meal, nausea, acid reflux, weakness. It is advisable for you to start looking for the source of it. You should have abdominal ultrasound performed and full blood work, urine analysis. Upper GI endoscopy is also advisable.
Your back pain can be caused by urinary tract
infection/stones, pancreas or gallbladder
disease or can be a result of what you have mentioned "funny sleeping". Abdominal ultrasound should rule it out.
Hope this will help.