I woke up this morning with lower side back pain just on the leftside. what could be the iusse?

Multiple Causes: Without a better history, it's not possible to say. Is there history of sprain of your back say from lifting or prolonged bending? Have you noticed any change in the color of your urine-could be a kidney stone. May just be sleeping on a "bad" bed. Could even be coming from your colon from diverticulits. You need to see a doctor to get the answer sorted out.

Muscles: Acute pain, pain that has not been there for long, usually goes away with conservative care. Most often triggered by sore muscle that go into spasm or become inflammed. Underlying reasons are numerous. Use ice, heat, 15-20 min each, stretch and rub. If it doesn't go away in a week, talk with your doctor.

See pain specialist: There can be several reasons for back pain including lumbar disc degeneration, muscle inflammation/strain, facet joint arthritis/irritation, sacroiliac joint arthritis/irritation, nerve irritation. A thorough physical exam and diagnostic studies will help determine what treatment will be best depending on your diagnosis. See a board certified pain specialist for evaluation.