I have a 9 month old baby she was feveris yesterday and we took her to the doc which we he found her tonsils was swollen and red he gave her antibiotics but after the 2nd dose she's getting mouth thrush this is the 2nd time now after using antibiotic?

It happens: Antibiotics kill off the regular bacteria in a patient and allows the thrush yeast ( present everywhere) to take over sites that the germs would otherwise control. Nine month old babies are not at risk of Rheumatic fever & antibiotics are rarely needed unless a test confirmed strep tonsillitis. Most swollen tonsils at that age are viral.