My Dog Just Had A Seizure, Her Limbs, Body And

My dog just had a seizure, her limbs, body and head were all stiff and shaking, during this she was salivating. There was a big puddle of water/saliva. After the seizure, she tried to get up to go outside and her legs wouldn't work, they splayed out from under her. What would cause this, she has never had this before. She is a 5 year old spayed lab, current on all her shots. She gets heartworm preventative monthly.