My BP did this about a year or two back. Turned out that the heat pad had stopped working and her humidity levels dropped a bit too low for her comfort. I thought it was a RI but couldn't get a vet to look at her until around the next week or so. So, in the meantime, I got a new heat pad and got all her temps and humidity back to normal and she stopped within the week and didn't need to go to the vet and has been fine since.

I agree with the above posts. Give it time and see if your snake is just stressed about something if you can't afford to take it to the vet but DEFINITELY watch her for next couple of weeks and if she continues/gets worse within a week, I would take her to the vet.

I would probably schedule an appointment right now to see the vet in about a week just in case she doesn't stop hissing so you have an appointment all set up and ready to go. Nothing is worse then trying to find a vet at the last minute! And in a best case scenario, you can just cancel the vet appointment a few days in advance if you see she is getting better.

I hope everything works out for you!