I think culprit was your attempt at fast charging and your charger either false peaked (ie thought it sensed a full charge), or the battery just did not like the rate you fed it (some RX packs just don't like a fast charge). If you want to fast charge in the future you should experiment with the charge rate until you get resonable results, ie if your charger quits after a few minutes, the battery is full or it has false peaked, depending on the condition the battery was connected.

Probably best to use the wall wart whenever possible though. Fast chargers look for the voltage depression that occurs when the battery is full and the current is converted into heat. The heat causes the voltage depression, so you are actually overcharging the battery each time very slightly. Also, a slow charge tends to balance the charge between the cells.

Having said this though, I also agree with the others think it would be wise to check the capacity of the battery by using a discharger, otherwise you won't know how much capacity is left. If the one suggested is too pricey, Futaba has one for $40.

Good luck!