Well, low B12 can destroy peripheral nerves, some of which are autonomic, and some of which are sensory. "Pins and needles" is exactly how a lot of people describe the feeling, although personally, I feel like bugs crawling all over me. Here's the Peripheral Neuropathy page on MedHelp. It's pretty informative.
http://www.medhelp.org/medical-information/show/74
As you'll see, BP and heart rate are controlled by these nerves as well, so your POTS could be from the B12 defficiency. I remember my neurologist saying that low folic acid means something too, but I just can't remember what. Anyway, you might want to bring this up with your neurologist.