In my opinon, the epilepsy drugs you are taking are not the cause of your increased incidence of catching colds. The reason is that, there is no evidence that Lamictal and Ativan decrease your bodies immune systems, and the evidence for Keppra is the opposite. Keppra has been shown to have beneficial effets for increasing your bodies immune system's effectiveness.
However, there is a possibility that your colds are really allergies. As we age, at different times we may develop allergies we never have before. It is kind of strange, but research indicates an increase in allergies at different stages in life. One source indicates these as about age 24, then early 30's then the mid 40's, and the 50's. Recent health news has indicated that there is an increase in allergies accross all age groups over the past couple of years in the united states.
If these are not alergies, it could be that you are just being more exposed than before to others who have colds. Your disease process could also be causing your body to be fatigued and therefore more susceptable to viral infections.