The Dreaded Measuring!

So as I was knitting away on the gusset, I was measuring it as I went. Don't want to go past those 39"! I took one measurement, 37.5". Success!! Continued to knit. Measured it again, only 35". Did i measure wrong or did this thing just shrink? Laid it down on a flat surface and measured. Definitely 35". Now at 36". Can't wait to get those last 3 stubborn inches!!!! Then going to start on the pocket. The adventure continues...