What Causes Tingling And Numbing Sensations In The Body?

the other day whilst in my husbands car, i got what was like numbness/tingling in the back of my legs from my knees to toes, it lasted a few hours, last night after being sat for 5 minutes at my slimming class the whole behind of me went numb/tingling/cold/warm, it hard to describe, it was from the top of my head all down my arms, back, legs, again it lasted a few hours, it's only on the back of me and it's very scary, please help.