Your cycle begins on the first day of your period and lasts 28 days. During the first 14 days, your body produces more estrogen, which tells the lining of your uterus to thicken and your ovaries to release an egg. In the second half of your cycle, the egg is released and your body makes more progesterone to prepare for a possible pregnancy. If the egg is not fertilized, the uterine lining is shed through your period and the cycle starts over. PMS symptoms are tied to your monthly hormonal changes.