This is day two of your period and day two of your complete menstrual cycle, which starts on the first day of your period and ends on the first day of your next period. A full cycle is, on average, 28 days, but many women have a shorter or longer cycle.
This may be the time when your period is at its heaviest, since the tissue and blood that make up the lining of the uterus (the endometrium) is shed. The average blood loss during menstruation is around two tablespoons (30 ml). While the lining is being sloughed off, the blood vessels in the uterus constrict, which can cause cramplike period pains. As soon as your period has finished, an egg begins to mature within its follicle in one of your ovaries, ready to be released around mid-cycle. This is called ovulation (see This is Day 14 of your Menstrual Cycle).
Meanwhile, the lining of the uterus starts to build up again under the influence of the hormones progesterone and estrogen, ready to receive a fertilized egg. If the egg is not fertilized, hormone levels fall, the lining sheds, and the cycle begins again.