Hand paresthesias can be caused by a variety of conditions. (For those of you that will be in the medical field- there's "glove paresthesia," in which the entire hand is numb, fitting like a glove. That means the patient is probably nuts, as there are two nerves that separate the hand, left and right- fingers on one side or the other may be numb, but almost never the entire hand.) The B12 is a quick and easy thing to test; get some sublingual B12, and pop it under the tongue. Double or quadruple the dose the first time; B12 isn't toxic in the microgram levels present in tablet form.
How's your meat intake? B12 has no vegetable source, so it *must* be acquired from animal sources. A strictly vegan diet will cause this condition.
Failing that, here's a longshot: your profile doesn't list any TNF inhibitors (Humira, Remicade, etc.), but there are reports of iatrogenic (doctor-caused) systemic lupus erythematosus from these drugs. Lupus *can* present in this fashion, but I would consider this an unlikely proposition.