Brief Answer:
Possible change in blood pressure reduced perfusion of foot

Detailed Answer:
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It is quite possible that upon standing you may have caused a SUDDEN drop in blood pressure which would've reduced the amount of the circulating volume to parts of your body. I would be concerned that because of that phenomenon what you felt in your foot was an indication that there was some type of low perfusion pressure state in the foot which caused it to go numb due to a vascular blockage somewhere in the distal vascular tree.

Or the feeling could've been unrelated to blood pressure considerations and simply been part of the beginnings of some form of NEUROPATHY tyou may be acquiring which you sensed as an odd slick sensation in the foot when in fact, you were steady as a rock.

I would ask my primary doctor to run the following blood work after something like this:

1. FT4, TSH, cortisol levels in serum, liver and renal function tests
2. Vit. D, D2, and D3 in serum, Vitamin B12, Mg, folate, ESR, and CRP.

These are all blood tests which are standard to get in situations such as this.

There are other causes of neuropathy which is the generic name given to any family of odd sensations such as what you're describing and associated often times with elderly individuals.

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