Frosty is used to getting her bottom poked by me - I don't let her back in the house if she's had an explosive poo (ate something bad and it came back to haunt her), without checking her butt for drips - with toilet paper. And sometimes she doesn't chew the grass she eats well enough and needs a bit of help with that.
But she was tolerant but uncomfortable when the vet took her temperature the other day. I think the vet was surprised she didn't have a go or try to sit down, but she did screw up her face a lot - if you can imagine a dog doing that. Same as when she smells something she doesn't like (eg vicks vapour rub).