Are the bubbles that come from the diffusion stone large or small? If they are larger, then perhaps you're using more O2 than I am-- my diffusion stone is 0.5 microns, offers a lot of restriction and makes very tiny bubbles. A larger size diffusion stone would offer less restriction, make bigger bubbles and use more gas. But even that may not make as big of a difference that you are seeing.
Another probability is a leak in your setup-- check the fit of the tubing against the regulator hose barb and the wand.
Maybe you left the gas on and didn't realize it? Or maybe your cylinder had a leak?
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein