That sounds like a cracked head. If you flushed the engine after it had been running for a while the sudden shock of cold water will split a head like its made out of chalk. Check your plugs and they will give you an indicator of where the failure has taken place (water entering a combustion chamber will scour all traces of carbon from plugs etc) The cold water from a garden hose going against the flow of hot water has enough pressure to overpower the pump. Rules are never try to flush an engine with it running. If it's at operating temperature there is a high risk of being seriously burned and any sudden changes in internal cooling temps can do the kind of damage you may have already experienced