Dear patient Considering your case it seems you have injured your back unknowingly as you were under effect of alcohol. Back pain while walking as you mentioned is very rare. So it might be muscular pain. Still you need to be investigated with Xray of lumbosacral spine anteroposterior and lateral views to rule out bony abnormalities. If it s normal take tab diclofenac 50mg twice a day for pain relief for 5 days and you will be fine. If xrays is abnormal take a visit to expert orthopaedic surgeon asap. All the best.