Have you had any physical therapy since your microdisectomy?
It could be that your muscles are weak from surgery and non use or incorrect use from the sciatica.
It doesn't take much to weaken core muscles and even though you think you are using them, when you had the sciatica, you where not moving, walking correctly and the muscles where not being used correctly.
Since your surgery and the sciatica pain is gone, you are moving again and the muscles are weak.
Speak to your doctor about a round of physical therapy to learn how to strengthen your core muscles, keep the strong and used properly, How to properly use the muscles so that you don't have any further spine problems.
Fran