Help! What the hell could have happened?

[CELA]

alright, so i've been looking online for all of the things that could have gone wrong. And I am still confused.
First time I smogged I passed everything except for the NO at 15mph.
As seen here:

So I get my car diagnosed at a different place cause it was cheaper, tells me that the theres a slow response from o2 sensor and poor efficiency from catalytic converter. So I try to save some cash and just swap the o2 sensor first maybe i wont have to put up the cash for the cat.
I change the o2 sensor, and reset my ecu, (but i was told that i didn't let it sit idle for long enough.)
As I drive the check engine light stays on, but eventually disappears. When I get some free time I head to the smog station, the light turns on again, and then goes away before I get there. I get tested, and the results are worse than before!
here they are:

The only thing I changed was the o2 sensor? Could the cat have gone that bad that quick? I'm thinking maybe I didn't reset correctly? Any advice appreciated