My cycle is always very regular and now ... What can it be?

Hello Dr. My cycle was always very regular, every day 20. It happens that in May my menses came little and darkened, in June came normal, in July came little and darkened, but for more days and in August has not yet come. There is no possibility of pregnancy, since there are times, times even if I have no relation. I feel every symptom that the mentation is coming, swollen and aching breasts, very mild cramps, headache. As the days go by, the symptoms pass, but the bleeding does not occur normally. What can it be? Thank you!