Have you heard of the nitrogen cycle before? My guess would be that is what you are experiencing, and it can be quite harmful to your fish. What is happening is that your fish waste and un-eaten food is being converted into ammonia, nitrites, and then finally nitrates. Ammonia and nitrites are really harmful to fish, so when you do get you test kit *most people swear by the liquid ones, instead of strips* be sure to keep a close eye on your levels. If your ammo is above .25 then you should do a partial water change.
Hope I didn't overwhelm you with this information. Also I believe that your tank is a little on the small side for that fish.
Welcome to AA by the way.