Dear thanks for the question.
You have got infection in
urinary bladder and urethra, which is common in case of stones. Stone in bladder acts as a focus of infection and that infection is causing you pain, not the new stone. However possibility of new stones is always there but this pain, decreased urine out put but urge to urinate frequently and
back pain are due to infections in bladder and urethra. Black stools however are not related to it, but it indicates some bleeding in GASTROINTESTINAL tract.
If I were your treating doctor I would have gone for urine investigation (microscopic, culture and sensitivity) , stool investigation (for blood cells and parasites) and
ultrasound of abdomen to locate stones and urine retention .
I would like to advise you to visit
urologist immediately to under go investigations and treatment.
Meanwhile you can take
norfloxacin 400 mg twice a day for a week if available over the counter or else get it prescribed by general practitioner. Keep taking lot of fluids.
I hope my reply will benefit you. Please write for follow up queries. Kindly provide feedback. I wish you rapid recovery. Thank you