Dress alterations/weight loss

Hi all!
So I went today to what I thought was picking up my wedding dress from alterations…. However when I tried it on the back was a little baggy! There was a definite space between the dress and my back itself ! The dress fit me nearly perfectly when I bought it back in January… even with my last fitting appointment a month ago the back was perfect ! My seamstress was just making final adjustments to the front, now the back needs taken in. I honestly haven't been trying to lose weight. I consider my body frame to be athletic and if anything I was just trying to tone my back a little more… perhaps I went a little to hard at the gym the past month 😅

Anyone else having weight fluctuation issues?