Dress doesn't fit

My wedding is in 3months, and I tried my dress on yesterday...it doesn't fit! I've been going to the gym and I have not gained any weight, so I dont understand. The only thing I can think of is I've gained some muscle mass to my back from the exercising, so maybe that's why I can't zip it? But it's probably a good 3-3.5 inches from closing. I have an outlet that I bought it from that would buy it back from me, and I could find an alternative. Or is getting it altered by that much worth it? Here's the thing, when I realized it didn't fit, I didn't feel devastated. I just felt stressed because I need something in a short time. Is that normal, maybe because of the stress? I'm wondering now if this isn't the dress for me and that's why I don't feel too upset. The thing is, my mother LOVES this dress and was so heartbroken when I told her. I'm torn, and could use some (positive) advice! Thanks in advance!