Haul Truck Operator reported that EE's lunch bag shifted as EE was getting into the Man Van at the beginning of shift and aggravated an old back injury. EE took pain medicine and EE's back was feeling okay. After the medicine wore off during the shift EE's back started hurting. NOTE: Reported back pain at 2:45 AM on 03/09/2023; shift began at 6:30 PM on 03/08/2023.