What Causes Spotting And Delayed Periods?

hi doc! i am 32 years old have 1 child who is 3 1/2 years old. i am on no contraceptives and ve missed a period for 8 days.have terrible lower abdomen pain and breasts are sore.i had taken postinor after having sex but the second tablet was taken late and it doesnt usually help. i had spotting on the 2nd day of missed period but it stopped there only.