If it were mine, I would:

1) Disconnect and re-connect the after cat O2 sensor and inspect wiring. Reset CEL. An intermittent or slightly dirty contact could be the root cause.

2) If it lights again, change all O2 sensors.

3) If it lights again, then consider changing the cat.

That is based on my experience of what the most likely culprits are on a vehicle with only 90K miles. It is just convenient for you that the cost of each is inversely proportional to the likelihood!