Engine starting to strain- but why?

Hi! I have a 2000 Contour, that just started to strain and feel like the motor's pulling too hard to run when I now go on the interstate. I'll be going 55, and can't get the car to go any faster, no matter how hard I try, and then the car will gradually climb up to 65, but only with a severe pulling and straining. I'm not really due for an oil change, and I don't even know if that will help, but I thought I'd try that first. Is there something I should be getting done on it at the same time? It only has 63000 miles on it. Thx!