Have you spoken to a GP? There are a host of reasons you could be lethargic. If meds are sedating, you usually know straight away when you start taking it or directly after an ncrease, but I'm no expert, never even touched Lamictal so don't quote me.

I very rarely know I'm physically ill until I'm really sick (which means I've been full of germs for a while already) but have noticed that fatigue is a huge red flag, for me as I always get really pooped out when my body is fighting an infection.

If your meds haven't posed a problem with fatigue before, and you haven't had a tweak that may have caused this, and you're sure you're not just worn out... I suggest speaking to your gp, it may be as simple as needing a vitiman boost.