Well, one way to look at it is that BCG has stirred up something! Let's assume it is your immune system, as advertised.
Seriously, BCG is the treatment of choice for CIS. And yes, your immune system has become sensitized to it. It is not unusual for the first few instillations to be relatively benign, and then for the side effects to increase as the treatments continue.
Did you have only a six week series or did you also have maintenance doses of BCG. Studies have shown that the six weeks and then a series of maintenance doses greatly increase the effectiveness of BCG.
If the side effects become too uncomfortable, your urologist can decrease the dose significantly, Even at 1/10the the original dose BCG can still be effective. By the time I finished my two year maintenance series, I was down to 1/3 the initial dose and this is not uncommon, You may want to ask your doctor about this.
Here is an interesting summary of information on BCG by Dr. Lamm, one of the experts in the field.
www.bcgoncology.com/treatment/bcg.html
Good luck to you
Sara Anne