Bleeding every two weeks

For the last 6 wks, every 2nd wk i get what is like a period lasts 5-8 days, it varies in colour, real dark almost black to light brownish, light to heavy i have bad cramps constantly like my period sometimes a little worse, i feel nausea have trouble eating, lower back aches I have taken 2 home preg tests both neg. I got my period the first month of my first (only pregnancy). Any suggestions? I thought i was pregnant then thought maybe a miscarriage. Now im so sick with worry i cant tell if im sick because of this or worry....