Does it occur at an other time? The dizziness during spell work can occasionally be explained by changes in your energy although it would typically only occur at the end of the spell and last a few moments and rarely during meditation. Does the usually occur after or during the trance? If during I would recommend trying to meditate lying down as our inner ear structure means that we do not get as dizzy when horizontal, if after then I would try waiting a little while standing up slower to give your body a little time to adjust.
It is possible that it is merely being stationary for a length of time is causing the numbness. Unfortunately if you've tried other positions I can't personally recommend much to fix the issue, other than flexing the hand during the meditation which can be distracting. It may be worth a trip to a GP's just to check it out although I'm not sure what they may or may not be able to do.