When I switched from divalproex (depakote) to lamotrigine a couple of years ago, I learned that lamotrigine can interact with other drugs. When taken with divalproex, it can double the potency of lamotrigine and is listed as a "major" interaction with it on drugs.com.

I did a quick search on lamotrigine and perphenazine and it is listed as a "moderate" interaction.
Here is a list of some the symptoms one should watch out for when on these two drugs.

Lamotrigine and perphenazine Drug Interactions - Drugs.com
So, it kind of looks like it may be the lamotrigine as you suspect.