"Is anyone else having bad abdominal pains about 8 hours after treatment?"
My short answer is "no."
I finished my last of 6 weekly BCG treatments on December 31, 2019 (induction phase). I was told to drink plenty of water immediately after the first void, which should happen 2 hours, but no longer, after the BCG is introduced into the bladder. My first and last treatments were almost side-effect free. Moderate burning pain, spasms, and bleeding occurred after my 2nd and 5th treatments. The worst of the side-effects, I'd describe as moderate to severe pain, spasms, and fairly heavy bleeding occurred after the 3rd and 4th treatments. With this said, in every case like clockwork, the negative side-effects subsided considerably by the 6 hour mark after first void, which is 8 hours after instillation. I've read that it takes at least 6 hours for all of the BCG to flush out, so it stood to reason that I began to feel MUCH better around the 6 hour mark.
Having said that, I've since noticed a deep, dull ache in the bladder area. It is similar to the muscle ache one experiences after exercise: DOMS or delayed onset muscle soreness. It's barely noticeable and not the severe pain and gas that you are experiencing. I'm not very alarmed by it, assuming it's the bladder still healing.
I hope your symptoms subside. Please keep posting. I learn something each time I visit this forum.