Could you stretch it out, if it is only a little tight?
Bring it to a taylor (I feel like that is the wrong spelling, but I mean a person who hems and changes the size of clothes, not Taylor Swift!)
If you are good at sewing and/or aren't afraid of messing it up, try to do it yourself with some extra fabric. I would start by cutting open one side and sewing the extra fabric on, but I really don't know how that would work out. Not well, I'm guessing.
Good luck =]