WHAT is wrong with my car? it won't accelerate..?

Recently I had two different tires on the front end of my car. I now have two tires that are the same size, but now my car is having trouble accelerating. I have a 2000 manual Plymouth Neon. It was in great condition until this happened. It struggles going up small hills, makes a very loud noise when I try to accelerate, when I p the hill and Im going more than 20MPH it sort of picks up until I have to stop again. what is happening!? PLEASE HELP!