It is completely normal for your laptop to generate heat whilst in use.
It is completely normal for your laptop battery to generate heat whilst being charged.
It is completely normal for your laptop to generate even more heat when it is being used AND charged at the same time.
Ultimately as something is used, it generates more heat, whilst you are charging, you are also using the adaptor and the battery, therefore generating more heat. As long as it doesn't get to the point where it is VERY hot, there is no problem. Just ensure your temperatures stay below manufacturers guidelines.