Probably, your partner is stimulating your G-spot. If you let go urination sensation, you will not emit urine but some white fluid. This may be small in quantity. However keep plastic to keep things clean. You will ejaculate with full body orgasm.To get more details google' feeling of urination before orgasm". you will find many website on this subject. Very few women are lucky to have this type of feeling- i.e discharging with orgasm.
Honey your orgasm is that weird peeing feeling your on your road to climax when you getting that ugly pee feeling.Dont be afraid go have sex and let the PEE FEELING TAKE OVER. you wont regret it
Just let yourself go. Holding it in is causing you the uncomfortableness.
throw down some plastic and pee....some guys like that