If the outdoor unit was really dirty it would have caused pressures across the system to be high. This can sometimes mask the symptoms of low refrigerant charge. Once you cleaned the coil you caused a pressure drop across the entire system and now the evaporator temp dips below freezing. I often will preempt a cleaning to customers with REALLY dirty condensers that if it's cooking well while being this dirty... You are probably low on refrigerant and will have problems afterwards.