Please Help me....?

My cat "Ellen" was spayed this week. When I brought her home I noticed that she had phlegm, because everytime she tried to purr, she was making a terrible noise, like she had a full chest. She started to cough and sometimes she would cough and vomit. She has been sleeping a lot too. I took her to the vet today and he prescribed me some antibiotics. I am so worried... what may be causing that? Anything originated from the surgery? Help.