Period was on time but scant and only two days. I've had cramping and fatigue. Back hurts tender breasts. Negative pregnancy test.?

Re do the test. You may wish to repeat the test in 4-5 days, with first morning urine. Or you could see your doctor for a blood test. See this site for more info. http://www.webmd.com/baby/guide/pregnancy-am-i-pregnant.