First of all, I wouldn't entirely trust a strip test - they aren't as accurate as liquid tests.
Secondly, you may get a false positive after a water change, depending on the conditioner you used and how much you stirred the substrate. Test it again in 24 hours.
Thirdly, a small amout of nitrates, up to 10ppm, is normal in a tank with no live plants. I don't know how many ppm caution or stress are equal to, though.
Hope this helps a little. :)