Transmission filter not all the way seated (fell down all the way to the pan) in the valve body and/or the O-ring cut or slipped out of position.
OR
You replaced way too much fluid and it attacked your hardened seals and has now caused a fatal leak on one or more of the many o-rings in your transmission.
Don't ever try to empty a well used transmission of as much of its used fluid during a change. The new fluid will attack your seals inside the tranny and cause god knows what failure if you have too much new fluid mixed with old! (depends on which oring fails) No way to drain the torque converter without removing it or spinning engine over with ignition disabled. Not a good idea at all!
On a vehicle that is not used for the dragstip or towing replace the fluid in the PAN at 100K miles or even more.
My brother in law that builds many GM autos a year for a living says he doesnt replace the fluid in his truck until his tranny needs a rebuild! (4L60E)
Him and I built my 4L60E for my Sierra (5.7L) and it now has around 117K on that rebuild. Never changed the fluid and I tow a 32ft 5400lb Travel trailer and sometimes my 20ft rinker with it!
That tranny is nothing seriously special other than the beast sunshell, kevlar 2-4 band, valve body updates, hi energy clutch kit. Thats it!