I suffered this way as soon as I'd finished my course of radiotherapy.
Fortunately for me, it was worse only for a relatively short time. Occasionally, I have been caught out since but certainly not as bad as it was immediately after treatment when I only had a minute to find a loo. Usually now it's a case that I should have found a loo earlier and not assumed that I can hold it like I used to.
You might find the Macmillan guides about coping with the side effects of pelvic radiotherapy very useful. They helped me.
Anyway, hope things improve for you in the coming weeks.