Employee lifted a bag of ore weighing less than 5 pounds. While putting this bag in a box located on the outside of the truck, EE felt EE's neck and/or shoulder pop. The employee refused further evaluation at that time. At 11:15 p.m. EE informed EE's supervisor it was getting worse. When employee returned to site at 1 a.m., the supervisor took EE to ER for further evaluation.