Check to make sure your car is free from any binding. Then check to see that your charger is at the same setting as your batteries. Some chargers won't warn you if the settings are wrong, and that could damage your battery.
If the battery has slight swelling it could still be used, but I would avoid doing so if you don't know the cause of the swelling. Take a picture of the battery and e-mail it to danny at SMC.
Tamiya 416X/ Tamiya TA-05 IFS/ Losi XXX-SCT/ HPI Baja 5SC/ Tamiya M05/ Tamiya F103/ Tamiya F104/ Team Associated 10R5O/ Kyosho IGT2/