My female that was coughing and lethargic got worse. When I brought her to the vet, it turned out she had a respiratory infection.
When the vet opened her mouth you could see way down in her gullet, it looked like yellow custard. (gross) That's why her stool was yellow instead of white.
She was given 5 days of antibiotic injections. Had to feed blender food at first, then she started to eat on her own. She still choked a bit when eating solid food but eventually that went away completely.
After the third shot she was already back to being alert and active. Took a month or so but she is now back to her old self.
Thanks for your input and glad to tell a recovery story.