Good comments above, here’s a couple other thoughts.
It isn’t that unusual to have to re-do a brake bleeding procedure after a few days. Air can get in the nooks and crannies or just dissolve in the fluid. In normal use the air eventually returns to the lines and causes a return to sponginess. Re-doing the bleed a second time often solves this once and for all.
It’s unusual for the brake pedal to suddenly become spongy without someone having opened the hydraulic line unless some brake part or another is on the fritz. Before the flush, when you first noticed the pedal was spongy, I presume you or your mechanic inspected the brake fluid level. Was the brake fluid level at that point normal or low? If low, how low?
With used components in the brake hydraulic system – master cylinder, calipers, etc – over time they can develop a spur on the surface of the cylinders. This spur is never encountered by the moving piston in normal operation, but during bleeding the seal can move over the spur and damage it. That could be another explanation of what is going on. If so, the part containing that seal will have to be replaced.
In any event db4690 has pretty much listed out what needs to be checked.