There could be a couple of explanations here. First I would recommend checking that it was a high quality replacement battery as poor chemistry could lead to poor performance. Second to check would be a loose connection but if the phone is operating normally then it would appear the board is receiving the right voltages. Third, it could be a faulty battery even if it is of oem quality, but that would also show itself as an issue by abnormal things happening while operating. Finally it could be because it's a new battery it has more useable capacity than an old one as battery capacity reduces over time, and the heat you are now feeling is because of the ability to charge and discharge faster. I would expect a new battery to generate heat while doing intensive tasks like watching videos or gaming and charging. If the phone is operating normally and charges and discharges fine then I wouldn't be concerned about the heat too much as while using the phone batteries and boards can get hot. If the phone is idle...