Car sluggish, sounding more muffled then normal

In my car, when I was driving home a night ago, it just
started acting sluggish and sounding more muffled.
The car has a flowmaster 40 series muffler, so I was
able to tell it is more muffled. Anyways, the car is
acting more sluggish almost sounds like it can't expel
the exhaust. It has no abnormal sounds from the
engine bay with a little bit of what sounds like grinding,
hard to tell even with the windows down. Engine has
oil, and is halfway in the hashtag on the dipstick,
because of a tune, it deactivated the diag for the cat,
so I enabled it and waiting to see what codes come up.  
But I do know this, the longer it's running the worse it is
getting, could this be the catalytic converter, or is it a
engine malfunction of some kind
Just know it has a new air filter, MAF sensor, and the
throttle body has been cleaned on the front and back
side, (all within the last 6k miles)