My guinea pig has a strange discharge from his bum!?

I've noticed that my not neutered male year and a half year old guinea pig has been having this occasional discharge of hair and a really stinky substance that has a musty smell from his bum. I thought at first that the source of the substance was his bedding which was carefresh and that he had been ingesting the bedding. However, I have recently changed his bedding from carefresh to fleece and he still continues to get this occasional discharge. However he continues to have well formed droppings and he still goes to the bathroom regularly...almost too regularly if you ask me ;) and so I don't think that the discharge is like backing him up. But I would still like to know if I should be concerned.