Employee went to an outhouse, slipped on the icy floor, felt like they pulled a muscle but not reported. At the end of shift on 4/9 they reported their back was sore due to not sleeping well. On 4/11 they said that sore back was due to the slip on 4/7. EE was taken to a clinic for eval. on 4/11, released, no restrictions, returned to work. Received physical therapy on 5/12.