According to the symptoms you reported only a theoretical presumption can be given. The nerves in the pelvic area sometimes receive a series of signals. You probably recognized these signals as vibrations of certain of the pelvic muscles. The signals were not long enough to cause a contraction. Also, when a urination occurs, the urethral sphincter relaxes and the bladder muscles tense to push the urine out. The signals that are sent to the bladder muscles might have somehow reached a sensitive area thus causing an orgasm.