Dress slightly too small

Hi Everyone,


So when I tried on the winning dress, it fit great so they ordered the same size as the sample. Just got the dress in and it’s slightly too small. Possible the sample was stretched out due to people trying it on. It has an illusion back with lace details and buttons all the way up and there is no way to let anything out. The lady told me to be careful working out as to not build back muscles. I’m already broad in the shoulders to begin with, anyone have tips on how to workout so the dress fits better without bulking up?
PS, I weigh the same as I did when I went dress shopping.