if i were you i would ak in the "ask humaworm" forum. parasites are notoriously hard to diagnose, there are literally thousands of them, and testing for each and every one is impossible. there is truly no way the doc could be certain its not a parasite.
if i were here i would do a good bowel cleansing program and also do plenty of probiotics and digestive enzymes. if she is on an Antibiotic she needs plenty of probiotics to go along with that.