Difficulty & decrease urination. Lower abdominal & back pain on right side after urinating. What could be the cause?

Multiple. Could be bladder infection, but with back pain on right sounds more like kidney infection or kidney stone. See if there is blood in urine. Increase fluids, sx persist, seek medical attention.

Flank pain. abnormalities of the urinary tract causing obstruction to the urinary flow can cause pain. Kidney stones, tumors or other lesions can cause obstruction. You should see your primary care doctor. If this persists would advise blood tests for kidney function and a kidney, abdominal and bladder ultrasound.