I have a rescued guinea pig who I think is about four years old but could be older. For the past couple of days he has been reluctant to move and stays in his cottage in the cage. He comes out everyday with his playmate (who is younger) for a run in a playpen. He does move when he's out playing but very slowly his back legs seem stiff. Is there anything I could do for him? He eats normally and seems fine in himself but has just really slowed up recently.