You may want to ask someone to place pillows behind you as well, to help support your back. (breastmilk.com)
The girls sit on the floor with a stack of pillows behind them and put their trays on their laps with their snacks, drinks, and cuddly friends. (starkidsproducts.com)
You can use pillows behind your back and behind or between your knees to help get comfortable. (pregnancy-info.net)