Hard to say why it happened so fast... however, if you add ammonia to take the tank to 4-5 ppm (allowing time for the ammonia to disperse before testing) and then have zero ammonia and zero nitrites within 24 hours, your tank is cycled.
You'd want to be sure your test kits are working correctly, of course, if you're getting unusual readings, but if everything checks out, and you're not showing ammonia/nitrite after dosing the tank, you're done.
Jim