The body can indeed be pretty distracting. And it can form some odd (and irrational) habits.
What I suggest is if this happens then end the meditation for a few minutes and do something else. Oddly, going to the bathroom might help. The urge for 'number one' can cause that reaction.
It might be simply a matter of the relaxation of meditation opening the bladder.
Regardless, find something simple and mundane to do for ten minutes or so. Once things calm down again, go right back to meditation. Repeat as necessary with determination, and do your best to not get frustrated or dejected. It is normal for the body... if inconvenient at times. It might take multiple tries during the same session but eventually the situation will stop and you will be able to continue with your practice.