I always have pregnancy symptoms before having my periods, what causes that?

Hormonal changes: Menstrual periods are induced by a series of hormonal changes that occur over the course of a month (usually), but these hormonal fluctuations can cause symptoms of bloating, breast tenderness, nausea, fluid retention(they are called moliminal symptoms, and are very normal). The non-specific symptoms of pregnancy can be mimicked by a lot of things.