It's possible to close up... but you're saying you can't go UP a size, right? That's different than losing width you started with. I'd worry if you can't fit a dilator you were using comfortably before, but it's totally normal that some women can't *increase* dilator size (with a new one) as fast as the surgeons recommend. You can't lose width simply by not increasing the dilator size; you just won't gain any more.
But if you are losing width below what you started with, I'd definitely contact the surgeon.
The real problem, for me, is that the pelvic muscle is a muscle... the more nervous and scared I got, the more it clenched up. So while it's easier said than done to suggest you relax, truly, the main issue is likely just that you're worried.