SES light

[FBody05]

About a week or two ago I put my new exhaust on an deleted my cat. I put an O2 Sim. on the rear O2 sensor and everything was fine...for a day. Well, the next day my check engine light came on so I went to Advance Auto and had them scan the computer and it got back the code P0133, wich I was told was the drivers side manifold O2 sensor. I replaced the sensor and light went off a couple days later. The next day the light came back on so I had Advance clear it and the next day it came back on again. My question is is it posible that the wiring is bad or maybe I have a bad PCM? Also is it just a cowensidence that this happened the day after I put my exhaust on?