A hardened seal within the master cylinder is what I'd guess. Or maybe it's simply wore out.
If you can quickly stab the pedal and it bites.. then I'd be very confident that's what is wrong.
btw, Seals failing a month or two after a fluid change is quite normal. Especially if the fluid was old and had debris in it. When you flush the fluid the larger particals get trapped in the seal areas and then act as an abrasive on the seals over the next few months.. then they fail.
The only way to prevent this is to either flush the fluid every couple years since the car is new.. or to never flush it.
I choose the first method.