I'm not exactly sure what you mean by "turning yellow." I'm presuming it's a api test kit but I have no experience with those. Anyways, test your tap water for ammonia, nitrite, nitrate, and post the results. I'm hoping for you that your water source is the problem here.
If not then something has crashed your bac colony because a cycled filter doesn't allow the nitrites above 0ppm. My guess as to what caused would be one of the following.
. The filter was cleaned recently
. New stock was added and you cycled to less than 5ppm