Both 생리 and 월경 mean menstruation. 월경 is a medical term. It is commonly used in medical documents 생리 is an everyday word. It is commonly used in colloquial language. 생리(生理) originally means "physiology". But, it is widely used as an euphemistic expression of 월경 nowadays.

Those have same meaning of menstruation. However, 생리 has some other broad meanings such as Physiology.