What Causes Nausea And Dark Spotting ?

Hi, I had bad nausa about a week and half ago, then went away. I got like a dark spotting 4 days before my period was due and when it was actually due (Saturday) I haven t had anything bar very light brown spotting. No period!! Then on the Friday (day before my period was due) I started getting flutters and movement in my lower abdomen, to the point I could see my stomach move. And this has been going on for 2 1/2 days now. This morning I did a home pregnancy test and it came back negative. what could this be, could I be pregnant??? And I am on the pill, take it at the same time every day. I m so confused and freaking out.