Imagine this; you are playing and happily enjoying what is going on between the two of you. As the scene continues you start to feel pushed to your limit, but you continue to play. Then, almost without warning, you hit a wall and (hopefully) safeword.
You've just hit a limit. It could be a temporary limit; one that will resolve itself another time. These are quite common if you have other stress or issues you are dealing with. Many of that can impact how you process sensations including pleasure and pain. The limit could also be a new-found hard or soft limit; an edge of play for you.
In these moments you will probably feel guilty for stopping play. You may question yourself and why you stopped when you think you might have been able to play more. It's not an easy thing to realize that you are not infinite in your ability. Remember that you are just as special even if you stopped play.
Encountering limits while playing is a different beast than if you knew about them prior to play. Your mind is altered by the natural endorphins and adrenaline running around our body at the time and you can react extremely to something that in your relaxed state wouldn't even blink about.
I'd like you to understand that it is okay to find limits you didn't know where there. They could be play activities that you never experienced before that you found you don't like or it could be a pain or sensation edge. Either way, it does not make you any less of a person or a submissive or a masochist. It actually makes you that much more unique and special.
Aftercare, when a limit is discovered, is similar to other aftercare but it might require more conversation or be analyzing what happened to understand exactly where the new limit is. You will then have to discuss if it's something you want to hard limit or if you would like to try pushing it in the future. So do whatever aftercare you normally do after a scene, but prepare yourself for possibly needing more; especially emotional support.
Finding a new limit is shocking in most cases. Not only in that you reached a limit but that you went that far at all. Not everyone gets a thrill by pushing their bodies to the limit. Already I know that I have limits I have not reached yet and one day I'll find them staring me down. And I'm going to be ready.
When you encounter a limit it may be time to review your limits list again, perhaps go back to your Checklist and see if there are activities you need to update related to your limit. If it was a pain threshold, you might have had a temporary wall or like I said before, it may be your edge for pain tolerance.
Whatever the case may be, make sure that you know that limits are normal, some can be breached while others are solid walls of our boundaries. Learn from the experience and grow in play and pain.