I took Lamictal many years ago for about 4-5 years. I never had dark patches on my face. However, I did have issues with blurry vision for a time (when my dosage was a little too high, I guess. Doctor lowered my dosage and I never had the blurry vision problem again, but it was super problematic. Had it for about a week or so at school when I had to climb up stairs...)
Definitely document them, take pics if you need, and have that ready for your doctor or contact your doctor's office if you're super worried about it. I suppose anything is possible as far as side effects these days and not everyone's body reacts the same way. It's worth getting it looked into.