Default Battery false readings?

I had one of those "BD" battery which was swollen and the phone was restarting once or twice a day. I paid 40$ for a new battery and the S/N starts with AA. Anyways, the battery has been acting weird recently (it's just one month old) it sometimes charges itself and sometimes the phone turns off when the battery's at 40% and when i turn it back on it shows 0% and if i take the battery out for a minute and put it back in it goes back to 40%. This started to happen a lot after flashing the russian kitkat firmware. Does this indicate another faulty battery?
EDIT: The battery is 100% flat and not swollen at all