Why has my l5 mo. old pug started to "mouth" and/or EAT his own feces if given the chance.?

I have a pug mix who has done this periodically and I've steered him away from it but am repulsed at the times he does, in fact, take up his solid waste into his mouth. Is this indicative of some disorder? It doesn't ALWAYS happen but I hope someone has some answers, before I spend money at the vet.