I don't know a lot about fasting. But common sense says that the fast was not an effective treatment for your sciatica.
Successive fasts might prove otherwise, but if you have to not eat for 17 days to achieve pain abatement I would say it's a fail.
Recommendations and thoughts:
There is a nerve bundle in the colon, the name escapes me at the moment, that sits near the upper rectum/lower sigmoid. If the colon is distended due to parasite infection, other infection, mucoid plaque, or general unknown inflammation, I think this would be a very likely cause or correlate for sciatica.
So perhaps enemas in conjunction with the fasting might bring better results, or lower colon oleation therapy/ panchakarma.
Suggest also castor oil packs on the lower abdomen, p/b Colon Cleanse shakes, triphala, parasite cleansing, chiropractic consultation with a colon literate practitioner.