EE HAD FINISHED WASHING DUMP TRUCK WINDSHIELD AND WAS CARRYING A 5# BUCKET, 3/4 FULL, BACK TO WASH RACK. WHEN HE BENT DOWN TO SET THE BUCKET DOWN, HE FELT A SHARP PAIN TO LEFT LOWER BACK, STATES PAIN IS A 9 ON 1-10 PAIN SCALE, NO RADIATION OF PAIN. EE WAS GIVEN PAIN MEDICATION VIA HYPODERMIC NEEDLE INJECTION. EE RETURNED TO REGULAR DUTY THE SAME DAY.