It is the enxiety of the consequences that made your heart race.
OP, there are strains of cannabis (eg.,sativa) which act as stimulants, however, you just said it that your heart started to race when you saw the cops.
(having said that, weed can at times make your hear race, and do other things though not every time-but I think this time it was the incident...and the cop was playing with you/insinuating so at to find out more)!
The cop was testing you with all his suggestions, since cannabis is not enough to be as severe on you or concerned, so he threw out some speculative suggestions, to see your reaction, and to built upon that.
The drop-back of the head, they do it even with just alcohol, to see how much intoxication you had-if you lose your balance or dizzy etc., he was just playing with your mind, so as you disclose more then he had on you, as well as testing reality of the situation-physically.
The part where he made you dig and bury it on the side of the road...(lol), was-funny!
Overall he seemed to be a decent cop!
Yeah I could kinda tell he's probably never tried it before so he kinda has to go along with what the police training probably teaches him.
But yeah he was pretty cool. He just had me call a friend to pick me up until I sobered up.