I replaced my Cat and Muffler 2 months ago and didn't have any problems until yesterday.
CEL turned on so i scanned it and it has "P0432: Check for a clogged or damaged catalytic connector"

 

If it was a slip on, I would go back and check the fit on the upstream side. If you didn't use enough exhaust sealant it could have separated.

Don't forget to post your year, model, details etc. helps people understand your problem better. Exhaust is different across the years.

 

I got that irritating code like 5x after I seafoamed my engine. Turns out the seafoam threw all kinds off crap and plugged my O2 sensors. If memory serves me right, P0432 = "Main Catalyst Bank 2 efficiency below threshold". Maybe this helps, just my .02

 

Did you do the O2 sensors when you did all the work? If you did what brand did you use?

 

Kinda what I was thinking too