When the front wall of the vagina is stimulated, many women feel a sudden urge to pee. It comes from several things being stimulated together: the G-spot area, the tissue around the urethra, and the nerve endings of the clitoris. It tends to come and go, which is different from a full bladder.
Some women release fluid at orgasm and some do not. Either is normal. It is not a goal, and not a sign of a better orgasm.
If it helps you relax, pee before you start and put a towel down. If the feeling is uncomfortable, or feels like a real need to go, stop and go. If you are curious what happens when you stay with it instead, see trying squirting.