What Causes Spotting And Abdominal Cramping Before Menstruation?

hey, ive been taking loette for about 1 and a half years now and i do sometimes experience light spotting before my menstrual cycle is due. it is currently monday and last friday i started getting blood on the toilet paper, a really dark red colour. i am due for my next menses on wednesday but i am still getting occassional spotting from thursday, really light and brownish in colour. i didnt take an active pill last night (sunday night) because i am due for my period this week. is this spotting something i should be worried about? i feel abdominal cramping too, exactly like i do just before i get my period.