Well... I'd look at the metformin for now. It can reduce B12 levels in 40% of the people taking them. So get a test for this and if the numbers are below 400, you need to supplement ASAP.
(doctors may say a result below this is "normal" but it is NOT).

I'd also test your blood sugar when you are feeling the pins and needles and/or numbness. Low blood sugar may cause this.

Can you tell me what else you take? Many drugs have effects on other nutrients, and may deplete common things. Some blood pressure drugs do this, ACE inhibitors deplete zinc.

Diabetics lose magnesium thru the urine. It is part of the process.
When magnesium levels get low, there can be cardiac effects. There can be Raynaud's (cold hands/feet), etc. Diarrhea can come from metformin itself. Loose stools lead to low potassium and MAGNESIUM as well.

Here is my magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html
(do not use magnesium oxide for your supplement-- it is a laxative--and is not absorbed).

Get the B12 tested first off. And start a magnesium supplement (based on my link). Then see how you feel.
There are other things to do, that may/will help, but you should do these first to see how much of an effect they have.