Please, get her to a vet ASAP. When I brought my kitten home from the humane society, he had a serious upper respiratory infection. I thought I was going to lose him. He's fine now, but he wouldn't be here now, without an antibiotic, an antihistamine, and L-lysine, to boost his immune system. Good luck with kitty!
Well if your cat is acting like it has a cold or something you can give it some benedryl I know its for humans but my grandmother does it all the time all you got to do is cut it into thirds that way you don't give the dog to much at one time and trust me it will make them feel better.
In my view your cat is having an infection of respiratory system (upper or lower?) but other conditions also needs to be ruled out. As you have said phlegm is also being seen means it is there for some time therefore she needs to have her diagnosis done and treatment started. It could turn out to be some minor problem like mild rhinitis but still it has to be diagnosed. See you soon
Cats are prone to respiratory illnesses so you should call your vet. They get pneumonia easily.