That seems odd. I don't know why the nitrates would have dropped so much. Test again tomorrow. If you get the same result, maybe toss in a little of whatever you used to cycle (you used fish food or something I think, right?) and test again in 24 hours. Your cycle is probably over, but if I were you I'd want to make sure.
My nitrites dropped like that... Idk what caused it to do that but I also had no ammonia... Nitrates was my bigger problem