I would go to the vet ASAP.
I experienced this same scenerio a week ago, a dog with a very very mild sounding congestion, and suddenly I now have 2 dogs with very HEAVY sounding congestion, one that had to be admitted in ER overnight for breathing issues 48hrs ago, and is still having difficulty expelling boogers from his head.
All 4 dogs are on heavy Rx prescriptions, and 2 are to be kept very calm/quiet - this has been expensive as well, and seems to be lingering.