Is it possible to stretch the stomach a month after gastric plication surgery?

Yes, depending how the surgery was done.During a plication, the stomach is sort of
wrapped around it self.If stressed too
early, the stomach and “un-wrapped” and return to its normal capacity.

Gastric plication has many disadvantages and frankly I don't think it is an effective weight loss procedure. The procedure is still investigational and I recommend a sleeve gastrectomy instead for any patient interested in plication as a weight loss procedure

You had an experimental unproved operation.  It involves imbricating the stomach with sutures.  Even nonabsorbable sutures eventually pull through the tissue.  It is inevitable that this operation will undo itself.  I don't know your surgeon or how the operation was done, but it sounds like it undid itself fairly quickly.  Perhaps he didn't place enough sutures, or put them in properly.