What Causes Light Spotting,fatigue And Fluttering Sensation In Lower Abdomen?

My period ended on the 12/26/14 and since then I have been experiencing urinary frequency, nausea, fatigue and slight cramping. I had some pink spotting the past two days. Today I noticed a "fluttering" in my mid-lower abdomen. I poked at it, it was solid and when I poked hard enough it moved away. I took two pregnancy tests and both say negative. What's going on?