Hissing

[Bogertophis]

I'm just guessing here...but since hissing is apparently something unusual for your snake, it might be that she had a hard time passing some urate 'stones' with her
long overdue defecation...so it just might be that her cloaca is a bit sore. I'd hiss too, I'm pretty sure. Most snakes hiss when they're not expecting or wanting
any interruption & interaction, which...let's be honest, doesn't happen in "nature"- they're solitary most of the time.

My most impressive hisser was a large BCI that I used to have: she was cuddly & liked to be out of her enclosure (she was sometimes quite a challenge to put back in!)
but every now & then when I wanted to take her out, she'd hiss up a storm. That was her only way of saying "I don't feel like being disturbed", but usually it wasn't an
option, I was trying to clean, so no, I didn't leave her alone. She was always in a coil, & despite her loud hissing, all I did was just gently pet her coils (not in her face!)
until she calmed down, which she always did. It was almost like she "forgot she knew me" & had to be reminded that she was safe with me. Never once did she spin
around to try to bite me when I pet her, & after a few minutes, I'd just lift her out & cuddle her as always. I actually never had one bite from her in all the years...

So no, I don't personally take a hiss as a "deal-breaker" but it's up to you & whatever kind of snake you're dealing with. I'm sure there ARE snakes that will bite after
hissing, & only you know your pet, but BPs are generally placid snakes.

And that's another reason snakes hiss: they know before we can see any evidence that a shed cycle is coming, so maybe it's just that.