I am not a uro, but my understanding is that BCG cystitis and interstitial cystitis are two totally different conditions. My first, tentative diagnosis (before the bladder cancer was biopsied) was interstitial cystitis and I did a lot of reading about it. It is a disease unto its own, and they do not know the cause.
In the midst of my last BCG series, I had burning, urgency, discomfort....and my urologist said that I had BCG-cystitis (a very mild case). It was not treated with antibiotics (I think it is due to irritation of the bladder wall from the BCG and not an infectious agent).
We delayed the next treatment until it healed....he did say that adding more BCG when the irritation was still there could cause a cystitis that could last a year!
It is possible that you have IC independently (I hope not). There is a LOT in the web about this condition.
Sara Anne