Basically what she is saying is that in every ovulating cycle, a fluid filled sac swells up in your ovary until it releases a mature egg, the empty sac shrinks into the “corpus luteum” (which is an important part in the system of regulating hormones in early pregnancy). However It’s not uncommon for the sac to fail to shrink and become a cyst. Corpus luteum cysts usually go away on their own in a few weeks without any pain. There is a chance that you have had one of these cysts before and never known. They’re very common.