It could be:
RIC 368ba: Commodus, AE Sestertius. 177-192 AD. 24.54 gr. M COMMODVS ANTONINVS AVG PIVS, laureate head right / TRP VIII IMP VI COS IIII PP, Minerva walking right, brandishing spear and holding shield.
http://www.wildwinds.com/coins/ric/commodus/RIC_0368ba.jpg
What is the weight and diameter of your coin?