Remember ovulation occurs around day 14, so from day 15 the egg is released.
After a follicle ruptures to release its egg, it closes and forms a what is called a corpus luteum. The corpus luteum secretes progesterone and a small amount of oestrogen. Progesterone causes your body temperature to rise slightly until the start of the next period.
Progesterone also acts on glands in the endometrium (lining of the uterus) causing it to thicken and secrete fluid. The thickened, secretory endometrium is made in preparation in case an egg is fertilised – the endometrium can feed an implanted embryo until a placenta has formed.
Therefore between day 16 and 28 the uterine lining will be preparing for the fertilised egg to be attached, thus it has to remain thick until day 28.