I'd be a little wary of the accuracy of your BP reading if you took it in the car!
What monitor did you use? I would NOT trust a wrist or finger monitor..... and if you were using an arm-cuff system, how did you support your arm in the car???
Did you wait at least a half hour after the 3-mile mall walk?
Lastly, have you been getting these high readings consistently over a period of time? Or was this a one-shot deal?