xerostomia (dry mouth) can be a side effect from both gapapentin and lamictal, So it could be one or the other, or combo effect. Dry mouth certainly can lead to mouth pain and thrush. I'd try using mouth moisturizers....these are available over the counter at a pharmacy. There are sprays, lozenges, toothpastes, etc. If these are ineffective for that particular symptom, I'd discuss with your doctor.