As Kathy said ..something in your tank caused ammonia to be created and the nitrite is eliminating the ammonia (before it is eliminated by the nitrate).
Can you tell us more about your tank ...it's age, size, bioload and equipment?
5 gallons a week is a bit much for a 30 gallon.
Anyway, with that much being changed I don't see how only the nitrites would show up. I would guess a bad testing agent.