What Causes Scanty Menstrual Bleeding Along With Lower Back Cramps?

Hi
I am a 41 year old woman with regular periods. My period was due last Wednesday. Wednesday afternoon I had menstral cramping and then I started to bleed as the normal start to a period. That evening I only bled lightly and nothing overnight. The next day my tampon showed only a bit of blood and by the third day there was nothing. I have done 6 pregnancy tests, the last one on Monday morning. All were negative. Yesterday and today I have (Tuesday and Wednesday) I am experiencing menstral twinges in my lower back and feel like my period is about to start but there is only a little red blood when I wipe.