Most people who take Fenben use the horse paste either Sageguard or Panacur. Or they use the cheaper goat version. There is also a purer fishbendazole on Amazon from Thomas labs. Not sure why, but don't know of anyone using the dog version. Maybe just cost but it could be there is something in the dog version that didn't agree with you.
Above all, don't panic. You will feed the buggers.