My mom said I was allergic to sulfa drugs as a child, so could I have outgrown this allergy 20 years later?

Hard to say. True sulfa allergy can trigger a life threatening response and persist for a lifetime.Sometimes the label is applied in childhood when an unrelated skin rash or symptom occurs & is not a valid diagnosis. I would not randomly try a sulfa drug.You could have specific testing to confirm the diagnosis. It may not be worth your effort, since so many other new meds are available to treat you when needed.
Immune tolerance. Allergy to sulfa drugs are not known to be commonly outgrown, however it is possible through mechanisms of immune tolerance. According to the reaction in childhood, your doctor might design the appropriate test that has the least risk of a severe reaction.