The "pop" you feel would indicate a structural problem. Hard to say over the net really. If it goes away in 3 to 4 days as you say and makes it hard to bend at the waist it sure as hell sounds alot like the SI (sacro iliac) joint. Avoid movements that aggravate it and definately work on strengthening the area with hyperextension movements (easy at first,then build up). I'd have it looked at if it's re-occuring.
NYC BIG MIKE