sore throat, constant thirst

My 7 year old daughter has had a bad sinus infection. She was prescribed Zithromax and it didn't cure it, so now she is on Omnicef. In the past, when she was under 2 years, she had an allergic reaction to it and swelled up with her hands and feet. We have had allergy testing done, to where they say she is not allergic, so the doctor prescribed this. She has been on it for 2 days and is now complaining of a sore throat and constant thirst. She also looks a little swelled in her lips, so I am debating what to do?