holy cow all that from changing the oil?
you obviously did something to it, not purposely of course but you hit something or jarred something,,,
wow the cat was cherry red,, you should probably kiss the motor goodbye if you continue to run it like this.
if raw fuel is dumping in tot he exhaust i would look at the spark plug wires, maybe one is pinched and not igniting the plug for that cylinder on the ingition side, then during the exhaust side the plug ignites again to fire the exhaust but it is firing the raw fuel and just cooking your enging,you also stand a chance of firing the cylinder on the upstroke forcing the cylnder and rod's down during an upward motion even though the exhaust valves are open reliving any compression, in the end you'll throw a rod if the engine is stressed or screw up the valves and rods.
in other words that cylinder is firing on raw fuel instead of the spent fuel thus causing alot of bad shit to happen. least of all is the cooking of your CAT.