You carb'd with CO2 and dispensed with the beergas/nitrogen right?
if so, yeah you have a leak. you can carb and dispense around 8 kegs with 5lbs of CO2. If you only dispensed 1 keg with 5lbs of nitro, there's a leak for sure.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10