I concur, find out the exact trouble code first, but don't ever trust an oil 'n lube place. You are better off not even letting them touch anything but the oil, and even then, checking their work!
Get your oil done and refuse any further services. Autozone will read your trouble codes for free while you wait.
Catalytic converters can fail from normal use like a large # of short trip driving cycles, or just rust out over the years, but often their failure is a sign of a larger problem. Larger may mean more expensive or it may just mean something as cheap and easy as a misfire caused by a bad spark plug - but in those cases you may still need a new catalytic converter too. I'm just saying it could easily be rusted out at close to 20 years old but if there's another cause then that cause needs fixed first.
Trying to rule out leaking gaskets I would check the coolant and oil to confirm they don't look like one is mixing with the other. Coolant in the oil can cause damage so it should be checked before driving around much more, but I would HOPE the oil change place would notice something like that and mention it to you.
If you're ambitious you could crawl under the vehicle and check for obvious (rust) signs of exhaust leaks and especially the catalytic converter, but frankly from a code it might as easily be just an O2 sensor... but get the trouble code and go from there.
