join canine epilepsy group on yahoogroups.com -- sounds like epilepsy since foaming at mouth
unless you think she got into something toxic on the walk and if so the liver enzymes will be elevated -- if epilepsy nothing will be off on the blood work -- if liver disease or toxin ALT will be high so let us know when blood work is back what highs and lows are so we can try to help. Seizures can come from other things as well but epilepsy is hard to detect so your vet is correct as one seizure cannot determine it so if blood work is perfect then i would suspect epilepsy -- the drug they try first is potassium bromide and if they need stronger it is phenobartal.
MAKE SURE REAL IMPORTANT NO ROSEMARY IN DOG FOOD , TREATS, SHAMPOOS ETC as dogs that are prone to seize will seize on Rosemary as my healthy dog had a petit mal after eating rosemary chicken so VERY IMPORTANT.
Did she get into anything on that walk??