For a device like these the batterys overcharge very easily because they take such a short time to charge, I have have my fitbit hr charging for more than an hour if I can, it only really takes like 15 minutes.
But it sounds like the battery might just have been defective, I'd contact fitbit and submit the device for RMA.
[http://help.fitbit.com/?l=en_US&cu=1&fs=ContactUs|Fitbit support page]