unless a huge water change was done on this tank after the initial cycle, and ALL nitrates removed via that water change, then nitrates would be showing up, as they are removed via water changes (your few plants IMO would not account for all the nitrates being eaten up). i'm wondering if your tank didn't cycle fully/properly in the first place, hence the zero nitrates now.
i just went back and noticed that you used tetra safestart. IMO none of the 'bacteria in a bottle' products work, and are just a waste of money. i think when you used the safestart/fish food in the beginning, you didn't end up with enough ammonia from the fish food to get the cycle going, and then the cycle just 'fizzled' away - the plants ate up the ammonia and the bacteria colony never did get established.