Water-only fast - Ankylosing Spondylitis

I am 33 years old and I have ankylosing spondylitis (is a chronic, inflammatory Arthritis and autoimmune disease).
I have already done several 3 day water-only fasts at the past (one year ago, for religious/personal reasons). During that time I had noticed at the 3rd day great pain relief.
Now (one year later) I found that several people try water-only fasts for similar health problems.
So I am thinking to try a 7 day water-only fast! (first I will stop NSAID etc. for at least a week before the fast).
Any suggestion or similar health problems ?
Thanks