Thank you for the responses - it didn't even occur to me that a snake could bite its own lip but it makes sense. I'll leave further mouth-opening to the vet when I take her for a checkup.

I'm probably being paranoid about RI's (too much time reading the internet and assuming the worst case scenario!) - her breathing's silent, no other signs either - she's always been a yawner so I'll just keep super close watch.

She's now chilling with her nose poking out of her hide, no more blood, so I hope all's ok. Sorry for my panicked newb tone before!