OK, the doctors did a heart catheterization and found some things that may be causing my problems:
1. enlarged right side of the heart (expected with my heart defect).
2. pulmonary hypertension (high blood pressure in my lungs and heart)...kind of amazing since my blood pressure is amazingly low otherwise.
Treatment so far has been medication; unfortunately, it could take up to 1 month for the meds to become effective, so now it's a waiting game to see if it works.
Thanks for all reading for your support and concern. You guys are the best.