What Causes Light Spotting,severe Stomach Cramps And Dizziness?

hi there! i started bleeding very lighty yesterday from the vagina like spotting (im due for my period next week, so its early for my period to start, i get it late then early). and got severe stomach cramps. i also could not sleep last night very well. everytime i get up to stand or sit up i get dizzy. i also felt a bit of nausea today but i did not vomit. went to doctors today and they said i was not pregnant. (i was sexsually active three weeks ago). would you know what this is?