Not wanting to worry you,and only posting this as you are going to the Doctors.
My DD1 was like this in January,as there was a sickness bug going around I assumed it was that.She was ill on the Thursday,I took her to Out of Hours,who thought the same.
By Monday she was no better,saw the Nurse Practictioner who thought it was maybe a UTI,gave me some Trimetheprim Antibiotics,wanted a urine specimen to confirm.As she had not been eating or drinking much and vomiting,didn't get one for 24 hour,but I now expected to see a change in 24 hours worth of antibiotics,but she seemed worse.
So I insisted on seeing GP on the Tuesday,and she listened to DD1's chest,muttered "unequal inflation of lungs",wrote a letter and told me to take her to the Paeds at the Hospital,they assessed her,and X Rayed her and she had Pneumonia!!
She stayed in for 48 hrs of I/V antibiotics,and then came home.At last she started to recover.
Make sure they check everything (the Nurse forgot to listen to her chest previously )It was very worrying.