Hi SKK,
I don't think you have a valid reason not to stop when you are flagged by the police. Failure to do so would put you in an incriminating position as the police will think you are running away from something.
And when approached by a police officer, it is always best to have your hands on the wheels where the police can see and reassuring them that you pose no threat to their safety and thus yourself.
As for the pee, I don't know man. Never had such an overwhelming urge to pee before.....