Why are My boobs are engorged before each pumping session, only gets relived after pumping.

Here's why you feel engorged, pump and only felt relieved after that. Hope you have a schedule session for pumping. You can either do one side pumping and one side latching or you can latch and pump afterwards. However do not empty your boobs otherwise production of milk will be more and your schedule for pumping will be more. Engorged also means your boobs full of milk and needs to be clear too. It is "telling you", it is time to latch your baby or pump.