Please read. about my staph infection?

monday i went to the doctor because i had a big red bump under my armpit. the doctor looked at it and diagnosed it as staph infection. he gave me bactrim and a bactrim ointment to put on it. this is my 4th day on the antibiotic and i havent seen any improvement yet....is this normal? it does not have a head and is not draining or weeping...im supposed to go out of town next friday. will it be better by then? please help!!!!