HI Carter,
Anytime you treat with medications that are essentially a chemical cocktail there can be adverse reactions. Different water parameters and pH and temp can all affect absorption of medications. Its possible that your fish is sensitive to one or more compounds in the octocil. I have no experience with that medication but we have seen sensitivity to various dewormers and antiparasite medications from discus to discus.
Your fish were not eating before treatment so I have to wonder what was going on internally.. was there any swelling? Or white feces before treating?
"She started spewing out white poop." This does not usually happen when you treat a fish for parasites.. it can happen if the fish is passing tapeworms. Do you happen to have pics?
See here...
http://www.simplydiscus.com/library/...apeworms.shtml
It can also happen when theres a bacterial infection and blockage that lets go.
Al