Why are my ammonia levels rising after tank has cycled?

I have had my tank up for 4 weeks, it is a 20 gal and I used the fishless cycling method, after 2 weeks of adding ammonia daily, my levels evened out, ammonia at 0ppm, nitrite at 0ppm and nitrate at 10ppm. I did a water change, nitrates read 5ppm and I added 6 fish on 2/17 to start, 4 Platies and 2 Gouramis. I have tested my water each day and all levels were good. Then, I tested a few minutes ago and my ammonia reading was 0.25ppm, nitrites 0ppm and nitrates 5ppm. What is happening?