Gas hard to push when accelerating...why?

[jesswinz]

2000 Protege. Gas pedal is hard to press when trying to accelerate especially when trying to go uphill. (Excuse my lack of car knowledge when trying to explain this) I didn't really notice it too much when driving in the city but when I was highway driving recently I noticed it much more. She was fine at low speeds but trying to accelerate to 100km/h was a little rough and then going up a decent sized hill I could go from 120km/h to 70km/h and she sounded like she was working hard but not revving overly high. Any ideas what the problem would be?