Do you use treated tapwater? If you're treating tapwater, many ammonia test kits register TOTAL ammonia, which includes the bound, harmless ammonia as well as the toxic "free" ammonia. This may be what you're seeing.
Your nitrates really shouldn't change TOO much unless you're ghost-feeding the tank (as you should). Then, I'd expect them to rise a bit.
Are you ghost feeding the tank? You need to feed the bacteria you have established or they will die back to whatever level of food is available to them.
There's another thing that can happen if you ghost feed a tank TOO much...you can actually stall the nitrogen cycle, however, I assume this isn't the issue.
HTH