What Causes Delay In Periods And Light Spotting?

I have always had regular periods unitl 5 month ago. suddenly stopped i dont take any medication/contraceptives and im not pregnant. So i went to the doctors and had a ultrasound scan and internal check. that night i went to bed and woke at 1:30am (dont usually wake) feeling sick and with headache so went to the toilet in case i was sick it lasted about ten minutes and then i realised i had bleed. Why did this happen and is it normal?