Maybe you got some bad gas which caused the ceramics inside of the cat
converter to melt and obstruct the exhaust? A plugged cat converter
will plug off your exhaust and make your engine run poorly and have no
power. Besides the fact that the cat converter could blow up.
Get it checked out as to why the cat converter became plugged -- running
too rich, etc.. Then you'll probably need a new cat converter because
yours is toast now. And they're expensive.
I had a spare truck that I loaned out to a neighbor so he could get to
work and back after his car blew up. He was putting cheap gas in it,
and the cat converter plugged up/burned up, he never told me, but
continue to drive it that way, and the gas mileage was down to about
5mpg and my engine was being put through high stress.
I got the truck back after a month, and he said "it runs bad and the gas
mileage sucks". Jerk. I took the cat converter out and beat the burnt
ceramics out of the cat converter and put it back on. Now the exhaust
was flowing freely. Wouldn't pass smog that away, but we don't have
smog checks in this area, and I only drive it like once a month to haul
trash/aluminum for a few miles. I'll put a cat converter on it soon.