What Causes Cramping And Light Purple Spotting During Periods?

Hi. I m having mild cramping with scant purple blood. I m 6 days late and took a preg test 7 days ago but neg. I ve been on the Atkins diet for 2 weeks. Could that be causing my problem? Also I ve been trying to get preg for 5 years with no luck.(im 35 and have a 16 year old) the docs can t figure out why were not getting pregnant. Do any of these tie in together?