2000 Ford Focus loss of power / acceleration

Driving home this evening I heard a noise which made me think I'd hit a small pothole, then I noticed I just couldn't accelerate away, though it would pick up speed slowly and I could hear a slight 'fluttering' and vibration which I would then 'drive through'. I stopped the car and switched off,and checked the engine bay but there was no sign of anything obvious wrong. I restarted the car which started as normal, it 'ticked over' OK and revved up OK but with a slight 'fluttering' in mid range. On getting home I checked over the car again,and noticed the exhaust 'popping' at the tail-pipe and a noise coming from the squarish box (I've no idea if this is a 'cat') second to the end. Is the exhaust responsible for this loss of power, or is it something more sinister?