Is it silly to buy something you love that is versatile (*ahem* LBD) in two different sizes because of weight fluctuations?

A few months ago, I bought a the Origami Sheath Dress (J. Crew) and can no longer fit into it. I suspect it may be due to bulking up from running. It's entirely possible that I'll lose the critical inch or two off my hips while traveling during the summer, where running in public in some parts of the world is a faux-pas. The weight gain is less than ten pounds, but makes a critical difference.

I've been stressed out about not fitting into the dress and suspect the best way to overcome the stress would be to buy the next size up. The next size up is flattering, too, just slightly looser in the bust/waist.

Was this is a silly decision? What would you do in this scenario?

p.s. I'm ideologically opposed to Spanx et al.