And for the people saying their nitrates spike every few days and they do waterchanges frequent that is due to you being overstocked, underfiltered, and/or you also probably overfeed. Learn to balance this better and you do not need to do waterchanges every 2-3 days. It is just sad to see the OP doesn't know much about fishkeeping and he is keeping fish that are not suitable for beginners.
You probably did the test wrong or you did the test too soon. I would be better to check the nitrates before doing a water change, then you will see if a waterchange is even necessary. The fact you get no nitrate reading now suggest your tank is not cycled or you did it too soon. So taking the test before will tell you not only that your tank is cycled or not, but also whether it is necessary to do a water change in the first place.
If you have a reading beforehand, do the waterchange, then take the test again you have something to compare to. Then you will actually have peace of mind rather than being freaked out by people on this site who make it out that 40 ppm of nitrate is a cause to have a heart attack.