menstruation

Naka exclusive breastfeeding ako sa 3rd lo ko. First time ko mag-bf. Then from what I have heard, pag breastfeeding, hindi ka agad magkakaroon. Pero just today, may blood na na parang 1st day of menstruation yung dating. Is this normal? Or meron ba talagang nagkakaroon agad even though naka bf? Magfo-four months na since i gave birth.