I would definitely bring up your post-adjustment reaction with your chiropractor. I have found regular chiropractic adjustments to be incredibly helpful, but I have not reacted well to some more heavy-handed manual adjustments. My chiropractor has told me that I am much more sensitive to adjustments than the majority of his patients (based on my body's reactions to adjustments). He does mostly manual adjusting but will also use an activator at times when I am more sensitive than "normal." He once performed a manual cervical (neck) adjustment as you described and I did not react well - immediate vertigo and vomiting. He will not adjust my neck in that manner anymore and instead is either very gentle with his hands or uses an activator. I do not know if any of this is due to POTS or not. I saw him before my diagnosis and was not always this sensitive. My body does tend to be more sensitive to adjustments when my POTS symptoms are flaring and we both suspect that it is the POTS that causes my body to react this way. Gentle adjustments though have been incredibly helpful for me. I would definitely bring this up with your chiropractor.
Honestly, I'd be more worried about my symptoms flaring, which seems to happen anyway every time I lose weight (without the use of weightloss products). Phentermine is risky for someone without POTS, I think you're asking for trouble if you take it...