Back pain caused by gas is usually due to trapped wind. Since the topical spray did not help, you can try an oral medication like simethicone to relieve gas, or a mild antacid. If the pain is actually muscular, paracetamol may provide relief. Please do not worry, as this is very common. However, keep in mind that left-sided back pain can sometimes be related to other issues, such as a muscle strain or kidney stones. If the pain is severe, does not improve, or is accompanied by fever or urinary changes, please consult a doctor for a proper evaluation.