Most people who are learning to control their pelvic floor muscles tense their belly muscles at the same time. This happens so often that the muscles may seem to be connected. They are not. It is very important to separate the muscle groups and to keep your abdominal muscles relaxed while you squeeze your pelvic floor muscles. When tightened, abdominal muscles raise the pressure in your bladder and actually make it more likely that you will leak urine.
Abdominal muscles tend to push urine out of the bladder instead of holding it in.
To avoid using your abdominal muscles, rest your hand lightly on your abdomen as you are squeezing your pelvic floor muscles. If you do, relax and try again. Be sure that you do not feel any movement of your abdomen. If you are, you are probably using your abdominal muscles. First, relax completely and notice how you are breathing for a few moments.