I found this. Its not very encouraging, but then it doesnt tell me WHAT dose they are going by...

In dogs, lamotrigine is extensively metabolized to a 2-N-methyl
metabolite. This metabolite causes dose-dependent prolongations of the PR interval, widening of
the QRS complex, and, at higher doses, complete AV conduction block.

In case your wondering all of those things are to do with the heart. But i still really would not panic as it seems like a very small dose.