Hello,
Welcome to the ehealthforum and I am really glad to help you out. In view of the description of the discomfort which you're going through, there are a few possiblities which need to be entertained.
Firstly, it could be a stone in the urinary tract. The stone can produce pain/discomfort in the left abdomen and the back. An ultrasound/CT scan of the abdomen can help in ruling out the same.
Another possibility is acute gastritis/pancreatitis. Serum amylase/lipase levels will help in ruling out pancreatitis. Please consult a physician for the same. I sincerely hope that helps. Take care.