Crikey you bet I am.
It can be a problem if a toilet break is needed of course but I usually manage to go without.
I know I would be panicking the whole time if I was out of site of the car.
I can't set the alarm on the car either as any internal movement would set it off - not sure if guinea pigs would but in theory they could I suppose.
I do try to disguise the fact that I've got animals in transit too by piling stuff on top if I have to leave the car.
Don't worry Lucinda, it's only natural to worry. We just have to take extra care. I never leave my dogs in the car either anymore, it's just too risky.