If you had stomach migraines as a child, will you have them through your adulthood?

Maybe. There is a tendency for these kids to transition to adult type migraines as they pass through ado;escence.
Probably. I presume that you mean periodic abdominal pain. The commonest cause by far is poor diet----- high calorie malnutrition or "empty calories". You are probably following the same diet now as in childhood.