If the operation was truly at L5-S1, this makes more sense. Three days in bed might mean that the surgeon intra-operatively had a massive tear of the dural membrane that was unrepairable and might have unintentionally injured the S1 nerve root. The headache is from the leak of the dural sack. Only when the pressure in the sack can be contained (by either repair of the sack or the CSF fluid being entrapped in a space outside of the confines of the dural sack-pseudomeningeocele) will the headache go away.
Normally for a massive tear, a diverting drain is used and bed rest for 4-5 days is standard.
This is a pressure and gravity phenomenon. If you filled a long but narrow water balloon (like the ones that are used by clowns to make balloon animals) and laid the balloon on it’s side, the pressure in the balloon would be low and equal at all points in the balloon. If you took this balloon and then held the end of it up and let it hang, you would see that the bottom section of the balloon would be wider than the top. This occurs as gravity causes the pressure of the water to be greater the further down you go to the end of the balloon.
The same principle occurs with a dural leak in the lumbar spine. When you are lying down, the pressure in the dural sack is equal from head to pelvis and you have no headache. When you sit or stand up, the pressure in the pelvis region increases dramatically and will cause the tear to leak which tractions the meninges and causes the headache.
Did you obtain a new MRI? This would give some good information regarding the status of the S1 nerve. Your tests that you noted “weakness of the calf, absent ankle jerk and a positive slump test” are all indicators of an injured S1 nerve.