Unexplained headaches and numbness. It?

In the past week I've had two episodes of numbness in my limbs. It is temporary, and subsides after moving them, but I still feel weak after. There is no tingling at all. The first time was while crying and emotionally upset, and the second time was accompanied by a headache not unlike an allergy headache. Both occured in my car while sitting, once when I was driving. It's a different feeling than being carsick. My limbs go numb when I cross my legs, lay on my arms, etc. but it feels different than this (no tingling, sustained weakness). During these two episodes I was not sitting in a manner that would obstruct my circulation. Could you explain what is happening and how to prevent it? I am concerned that this may be dangerous if it occurs again while I am driving.