COWORKER ASKED EE TO HELP HIM STACK STRAW BALES ON UNEVEN GROUND. THEY STACKED BALES 5 HIGH. ONE BALE FELL OVER AGAINST EE'S UPPER LEFT ARM, CAUSING HIM TO FALL TO THE GROUND. EE ASSURED HIS SUPERVISOR THAT HE WAS NOT INJURED AND RETURNED TO HIS SHIFT. THE NEXT DAY EE COMPLAINED OF BACK PAIN, AND WAS SENT FOR MEDICAL EXAM.