discharge nose

Hi, I have just noticed my guinea pig gingy continuesly sneeze a few times in a row and clear discharge leaking from only 1 nostril. He is not lethargic, doesn't have crusty eyes or nose, just wet and runny, only when he sneezes.

How can you tell if your guinea pig has a respiratory infection? My boy has some yellow/green dried up snot on his left nostril (never the right one). He also has a cut under his left nostril by his lip that looks a bit swollen, I’m not sure how it happened but it appeared yesterday morning...