New Exhaust=Code 21

Hi! I have a 95 Grand Cherokee with a 5.2.
I just replaced the cat converter and muffler.
Now after about 5 minutes of driving, the check engine light comes on
I pulled a Code 21 for the oxygen sensor.
I don't understand why new components downstream could cause the Oxygen sensor to trip.
Thoughts?? How can I fix this?
Thanx!!!