Maybe you cleaned a part of the filter ?
It means your get maybe bad reading for ammonia or nitrites, or that you're getting a mini cycle.
What's your nitrite reading ?
Maybe you have enough NH3 BBs but not enough NO2 BBs to complete the cycle.
Also, it is possible you got a small ammonia spike without noticing it, and now you're on the nitrite phase. Do a 50%PWC just to make sure.
Also remember to never clean the filters media under tap water.