My dog bit me, I'm so shocked

[Snowedundersnowedin]

He's nearly 5 and we've had him since he was a puppy. He's never bitten anyone ever, although he gets very anxious and stressed at the vet and has to be sedated for them to ever do anything...but at home, and out and about, he's the friendliest, most sociable dog around.
I'm wondering if he's not feeling well, as he came in from the garden earlier with crap all round his bum, stuck in his fur, just yuk, and I had to bath him to get him cleaned up.
I'm going to phone the vet in the morning to get some advice, but I'm actually in tears here - not because it hurt (it did), but because I'm so shocked

Thank you everybody, I phoned vet this morning and she's visiting this afternoon...she suspects he has infection of some kind and that he might be in some pain. She isn't coming til later, but prescribed something to settle his stomach that I picked up earlier. So he's had that, and seems perfectly fine.
I've been to gp and had tetanus jab - last one was 1998 - and got prescription for antibiotics too. Dr said it was quite deep, and because its on my face she wanted to be on the safe side.
He wasn't asleep, and I don't think I surprised him....but who knows?
As for his eyesight, he can spot a treat at 100 yards, so I don't think there's a problem there!

glad the vet is coming. get her to give him a really good check over, doing his hearing and his eyesight (you might have to bring him in though for that). she probably will anyway.

lets hope he's just feeling a bit under the weather with his tummy and that he'll be fine once he's beter.