My iPhone is keep restarting and over heating after battery replace

I replaced today my iPhone's 3GS battery with the ifixit replacement kit and I have noticed the following:

- My device is restarting every 2-3 minutes

- After restarting it doesn't promt me for pin code

- It's over heating compared to my old battery (which was the original one)

- The new battery is being charged very slowly (e.g from 14% after 30 minutes the progress was only 7% and now it's at 21%)

Any ideas?

Update: I opened the case to check if I had forgotten any connector and after that, the battery indicator showed again 14%