Could it epilepsy, or antiepileptic drugs be responsible for gerd and difficult defecation?

Can be med effect: Many of the epilepsy medications can cause stomach irritation and sluggish bowels. Slow release forms can help, and attention to diet could help. Take medications with small amount food. Try not to take med with antiacids since this may interfere with absorption. Also be careful about citrus drinks especially grapefruit that can irritate stomach and alter absorption.

Yes: Check the adverse reactions o th medications which can be found in the "package insert" or check with the pharmacist. Just because it may be listed however doesn't guarantee the cause and effect relationship, but its a start.