You might have put in some low-quality petrol that may be causing your car to shudder and run poorly. Some cheeky gas stations "water down" their gas which is harmful to cars and gives poor performance. Once you need to refill your car, fill it up with some quality petrol...avoid the last place you filled up at. At the same time, I would go to your local auto parts shop and pick up a couple bottles of fuel injection cleaner and put one into your petrol tank with the next fill up. Do that for the next fill up as well and it should clean up your injectors and your car should be running smoother.