Is your doctor an endocrinologist?
Normally one would run a battery of other testing before going to the stim test such as cortisol and ACTH so you have a baseline (more than once, at 8am), sodium and potassium, renin and aldosterone, (those relate to the low blood pressure) as well as other hormones so you can find out of the source is adrenal or pituitary.
The POTS (dizzy spells) may or may not be related...
Let us know how the testing goes.
Check for Celiac blood work and endo scope. You wont regret it.