I just hope it is not the multires infamous “multires spike bug” that just popped up on your multires sculpt.
The multires spike bug is a very bad bug that happens sometime (and what makes it hard to fix is that it does not happen often or in a reproductible way)
If it is that nasty bug even when you fix it on a multires level, it will re-appear on another one.
A possible solution would be to use a previous blend from before the bug appeared , or rebuild a new multires sculpt from your base mesh (and shrinkwrap it to your buggy one to recapture your current sculpted details)
Psy-Fi, that may work on improving multires in the future, requested people to report occurance of multires bug like this one with blends so he can have cases to study.
I reported one (a variant of the spike bug in which the spikes are “bumps” ) , in case this is the spike bug, i suggest you to report it with the blend too :