Bulldogs World

Cartman hasn't been showing any signs that he's feeling bad, but he has sneezed a thicker, yellowish snot a couple of times.  The first occurrence was around last month and then again today.  His nasal discharge is clear.  There's no coughing, reverse sneezing, and hardly any regular sneezing at all.  The two times that he has sneezed like this, the snot is clear around the edges with yellow phlegm mixed in.  Could this be allergies/sinus issues or a sign that an URI is on its way?  I'm debating about taking him to the vet since these have been very sporadic episodes and he's otherwise his normal self.