You need to find her a vet. It is very possible she has a respiratory infection. They are very common in cats, especially strays. Please drop her off at a no kill shelter or take her to a vet. If left untreated she can get pneumonia or if it spreads into her eyes she can go blind (actually quite a common thing in strays). If you have another cat, please isolate this one from yours, make sure you wash your hands in between contact, and clean the area the cat is in well once the illness is treated. If it is a respiratory infection the cat is highly contagious.