Left foot braking adds another variable because it is easy to lose sensitivity in the left foot when switching back and forth between the clutch and the brake. If what you described happens when you are braking with your right, what it sounds like is that the brakes pump up -- the pedal is initially soft, then goes hard after being pressed first. This implies air in the system and/or a leak.
To exclude the master cyl, with the motor on press the brake pedal while at a standstill. If it sinks to the ground suspect the master. Otherwise, check fluid level and if low, suspect leak (or worn pads of course).
Another possibility would be that you are seeing the effects of hydrated fluid heating and cooling, but it's "only" autox and if your fluid is relatively fresh I'd not go that way at least initially.
Since you've just started left footing though that's suspect in and of itself.