Why would the doctor prescribe Sulfamethoxazole W/ Tmp Susphit for my daughter's stomach virus?!?

I took my 4 year old daughter to the doctor on Thursday with a stomach virus (constant diarrhea) and the symptoms got worse after taking the medication. After my mother suggested that perhaps the antibiotic (why would an antibiotic be given for something presumably viral and why the hell would I actually administer it is beyond me) was giving her worse diarrhea I looked into the side effects and taadaa! A common side effect is none other than diarrhea. Upon further research of the drug I saw that it's used for strep and flu and others illnesses of that sort so why would a doctor prescribe this for diarrhea...especially to a 4 year old?!? If it wasn't Saturday I'd ask the doctor myself. Anyone able to defend these actions? Anyone have a good reason why I shouldn't quit using this medication? The first dose I'm skipping is tonight's dosage with no plans on giving her anymore. I believe my daughter has had to suffer explosive and painful diarrhea 3 days more than needed and I'm pissed!