Please provide age and general activity of the patient. Backache
may occur because of over exertion or some kind of strain which relieves by rest and pain killers. Pain killers have an effective duration of 8hrs-12hrs, so that means after the effect of medicine pain was felt again. A consultation with an orthopedic surgeon will be useful, it probably may be a muscular spasm, so do not panic.
Wish you a speedy recovery to health