But what water did you put in it? Unconditioned water straight from the tap? And I know my local Wal-Marts have those cups that have a really deep cone-shaped air hole that goes far down into the cup. If you filled it up above that, then there's no way for fresh oxygen to get into the cup once what little is inside is used up.
I hope everything is okay, though, and I applaud your good intentions. It makes me feel bad whenever I see bettas waiting in their tiny dirty little cups for homes.