Two weeks shmooo weeks. Call your GP doctor back and tell him to explain it to you.
Underside could mean outside the bladder on the bottom. If that, then likely it has nothing to do with cancer. Bladder cancer typically starts inside the bladder. However, who knows what the doctor who read the CT was thinking when he/she said underside.
Chances of you getting to join our esteemed group don't sound so high based on that interpretation. That's ok though because nobody really wants to join this group anyway. We're a pretty neat group of folks but if you stay out of the bladder cancer (or any cancer really) club, then it is certainly the recommended option.
If anything is growing abnormally around your bladder, you go to a urologist to get that looked at so you have the correct next step in process.
Don't get too far ahead of things with trying to self diagnose using the internet. Typically this is not a good idea.
Best of luck!
Mike