What Causes Diarrhea After Having Antibiotics For Vomiting?

Question: My 22 month old son became ill Tues night, vomitting every 15-20 minutes for four hours. We took him to the peds ER where they gave him Zofran and sent us home. The next day he seemed to be on the mend (no more vomitting) but by evening was refusing liquids, food, Popsicles, anything, and was noticeably bothered. He then had very bad diarrhea from thurs-fri and was acting lethargic with few wet diapers, so we went back to the ER Fri to check for dehydration. They said he didn't seem that dehydrated and sent us home. Today is Saturday and the diarrhea has slowed down from a few times an hour to 3 times all day today, but he recently started vomitting again. Is this a normal progression for a stomach virus in a toddler? I became ill on thurs night but was fine by Friday and my mother who has been helping out was ill last night. I'm petrified of our son becoming dehydrated, we were in the ER before 6 months ago as he became severely dehydrated in a matter of hours and his veins were collapsing, it was a very tense and frightening few hours with a subsequent hospitalization. He's only wetting 2-3 diapers a day but the ER dr said that's normal as he will have decreased urine output, but that he still had tears and good vitals so there was no indication he needed IV fluids. Do we need a second opinion on this? He was acting normal although cranky before the vomitting, and is now sleeping. One big vomitting episode today, approx 1.5 hours ago. No fever. Was ill with cough/cold/inflammation in ears/fever one week prior, took augmentin for 10 days, first GI symptoms began 5 days after antibiotics.