Why I'm feeling stiffness in my ovaries after ovulation?

Mittelschmerz. There are several explanations for the cause of this pain. Just prior to ovulation, follicle growth may stretch the surface of the ovary, causing pain. At the time of ovulation, fluid or blood is released from the ruptured egg follicle and may cause irritation of the abdominal lining.
Likely mittleschmerz. Many women feel small pains around ovulation, before-during-or after - its called mittleschmerz. There are some abnormal causes of pain around ovulation but most often it's normal. Individual women choose different words to describe it, you say 'stiffness' - many in my practice describe 'pinging' or 'cramping.'.