2000 Fiat Punto losing power when people in rear of car

My car has just recently had a complete new exhaust fitted, since then ive noticed that when two people are sitting in the rear of the car, the car struggles when setting off/slowing down or when I push my foot on the accelerator, like its losing power, to me it sounds like its coming from the exhaust. The car is fine as soon as its gained speed and I am maintaining that speed and also fine when its just me in the car or a passenger. I thought I may have overloaded the car, but I have never had this problem in the past, any suggestions, could it be how my exhaust has been fitted?