I went to the shop where I got my unlock and the guy said it is a battery, not the phone. He said I apparently overcharged it and that the maximum charging time for this phone is about 3-4 hours?
I also visited Samsung Customer Service and they said I have to send them the phone and they will only replace the battery for free IF there has been something like the battery leak etc. Otherwise I will have to pay for the post + the battery....
EDIT: I have downloaded an app called Battery Dr. and on the battery health it says that there has been an over voltage - what does that mean?