Many people sail through BCG and report a bit of discomfort but no real issues.
Others have a different story. My problems started about where you are now. By the sixth BCG instillation, I could count of losing 3 days to just dealing with the BCG side effects. The first 24 to 36 hours were the worst.
I kept going because BCG is the most effective initial treatment for bladder cancer. I kept mine under control for about 2 years, with additional BCG treatments. The cancer would be gone, then reappear, so we stayed with BCG.
It is important to you talk to your care team about your symptoms, and the side effects you experience. There are a number of things that can help. Ask about pyridium (a urinary pain reliever) and other medications. Doses and timing might be varied to help you recover between BCG instillations. Hold nothing back - talk about how BCG affects you and for how long.
On your own, find out what helps you deal with the side effects. For me lying down and taking all pressure off the bladder was very helpful, as were hot packs over the bladder area. Hot liquids, once urination was possible helped a lot ( I like coffee, some docs don't - so ask). Lots of cold drinks, including carbonated helped (same need to see what doc thinks).
Do try find was to deal with YOUR symptoms. Ask for help.