Commonly known as by passing. Very common particularly when passing a motion, it can be slightly uncomfortable. No need to worry, unless you are passing great lumps of blood and clots out at the end of the penis.
As regards your twoc make sure you are hydrated the day before and on the day, then follow the instructions of your nurse. Stay relaxed and you will be fine.
Make sure you have some pads and take a towel and seat cover for the journey home in case of an accident.