Once again i have to guess a large amount here........
By cystitis, you mean going to pass water frequently, urgent need to pass water and pain as you pass water....not just pain in the stomach.
I am also going to guess that before you were put on antibiotics, no urine culture was done to identify any bacterial invader and if I am wrong, then the rest of my answer would appear to be invalid.
You urine needs to be tested for organisms and if present the organism tested against certain antibiotics and the appropriate antibiotic given.
If despite this, you still get recurring attacks, then you need to see a Consultant Urologist, who will investigate your "plumbing" more fully.
In all probability there will be nothing sinister behind this.