Hi Hui Yi,
The monthly period pattern is maintain by network of regulating hormones between the brain, the ovaries and the uterus. Disturbance to the hormonal axes may tweak the timing of the periods, the pattern of bleeding and the flow of the content.
If it occurs once in a while, it is considered a harmless disrupted state, and will probably resolved by itself. If it happens repeatedly, then a visit to the gyne will be appropriate.
