Gas hard to push when accelerating...why?

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?