Some resources list "concentration problems" as a rare side effect of Lamictal, generic name Lamotrigine, so it is possible that your memory loss issues could be caused by the medication. This is most certainly something you should discuss with your doctor.
Following are some additional resources for information on this medication:
Lamotrigine [Lamictal] - Side Effects
Voices of Experience: About Lamotrigine [Lamictal]