High revs low speed
 

I have a 2007 Monte Carlo LT and recently its been acting really buggy when I step on the gas it will rev high to about 3000-5000rpm while only doing 20mph I recently had the catalytic converter done and I have to do the Upstream O2 Sensor I haven't changed it only the Downstream O2 Sensor was replaced I figured this was my issue I checked my trans fluid and it's where it needs to be no discoloration nothing like that my instructor said he believes its trans related like a torque converter issue another instructor believes it could be something engine related however the car has no check engine lighthttps://cimg8.ibsrv.net/gimg/monteca...249312401c.jpg