It's probably radiation cystitis, where the radiation has burned the urethra or bladder lining. Things like this can take a few months to go. They would not normally investigate unless it's still there 3 months after RT, which is why a 12 week followup is normal.
One thing you might want to check is if you have any urinary infection, which your GP nurse could easily check with a dip test in a urine sample. Drinking cranberry juice can reduce the chance of a urinary infection because it coats the bladder and urethra with a chemical which prevents bacteria sticking. Ideally use a low sugar cranberry juice as it otherwise has a lot of sugar in it. I tended to dilute it 1:3 anyway, to make sure I was drinking a lot of water, which is also important at this stage - you want plenty of dilute urine to keep bladder and urethra well flushed through.