What is this?

My 5year old spayed indoor cat has developed a drainage coming out of the right side of her nose. This only occurs out of the right side. Her breathing is normal,she is eating the same. I 've had to wipe her nose for the last week. The discharge is greenish black in color. what could be causing this? Mollyhale

Have you had her into the vet yet?  That would be my first step.

Molly is going on Monday. There isn't any swelling of glands or anything.