This is very common and very much part of the routine. I was so happy be to done with the early morning water drinking routine. More than once I trotted to the bathroom after treatment, shoes in hand, not wanting to wait to put my shoes on.
They declined to give me a specific amount to drink. They just said have a relatively full bladder, not bursting. I figured out the water drinking timing the best I could but I had some close calls if they ran behind. Once I "had" to go just before treatment because of an equipment delay. They told me to let "just a little" out -- tough, but I did it! I knew my kegel exercises had paid off at that point. One part of the challenge is your bladder begins to react to the radiation so your capacity/urgency starts to change during the treatment course. (I started getting up 2-3 times a night to go.) If you are struggling you should cut back until you are full but can reliably make it through your session. Your bladder should be full but not bursting.
If you are having imaging done prior to each session, they can give you feedback on how full your bladder is. One time they told me my bladder was "huge" after imaging. They were impressed. So, I knew I was getting the job done.
The good news: my bladder was back to normal within 2 weeks after the treatments were done.