I have been on Lamictal for a little over 2 years, have had no problem with side effects, maybe just mild dryness around my neck, but then I am pre menopausal too, so that may be part of it.
Your mouth sore can be from certain foods, I know some hard candies can cause that too.
Have you asked your pdoc if this is from the med or foods?
Lamictal when started gradually, doesn't cause problems for most of the people using it, close monitoring should also be part of the introduction of any new psycho pharmaceuticals medications to help assure pdoc and patient that it is working or not, etc.
Well, I won't ramble but tell the pdoc about the sores, it may be diet and stress, sometimes that stuff can be nasty on our systems including change in saliva's PH, stress can cause more acids to be pumped into our saliva thus causing or contributing to mouth sores.
All I know lollipops, sour hard candies, foods, can cause me to get soreness on the roof of my mouth, some candies, I swear cause paper cuts in the mouth, ugh
You may want to give this mouth rinse a try, it is called Biotene, it is soothing and helps with salivary balance and germs, they also make a gum too

Well lots of luck with this, let us know how you make out, okay?

Sincerely,
DE