Which watch do you have? Are you using Move IQ or actually recording this as an activity? If recorded as an activity, after pressing start the timer will not stop until you press stop. Then select save.
Also, if you do not use MoveIQ, but rather a manually started activity, check the settings of the concerned activity (I assume the Run), and disable the option Auto-Stop, if it is activated. Although by default it should be disabled, I think.
