Liz Buchanan BVSc MRCVS
That sounds like a scary thing to be witnessing. It may be behavioural, but I think the concern is that it could be neurological or neurological secondary to something else (for example, low blood sugar or liver changes might effect the nerves). I can understand why, after a single incidence and then everything returning to normal, a vet might suggest that a toxin had passed through the system. But as it is now happening increasingly often and getting worse, this theory might have become less likely (unless, for example, they are in a position that they might regularly find drugs on the floor) and other suggestions, such as fitting or even a lesion on the brain, might be more likely than they previously seemed. We would strongly recommend going back to your vet with the new evidence of more fits. If it happens again in the meantime, videoing the incident may provide good clues and it helps to try to figure out whether you have some warning before it happened and / or a change in behaviour afterwards. In the past, if a dog didn't lose consciousness then we believed that neurological incidents weren't really 'fits,' but actually things aren't so clear-cut as that. Your vet might also want to check bloods to check for underlying liver changes etc. or to carry out a neurological examination. Where finances are available, neurological imaging can sometimes be helpful too.