please help!!!

AF was supposed to be here Thursday, instead I have lightly spotting (dark) where I don't need to wear anything but a panty liner. This is not normal, not to mention that it is paired with cramping on my right side and a tender tummy (I feel sick when I lay on my stomach), tightness in my chest/breasts. 
Took a pregnant test this morning which can back negative. I'm worried because I never feel this way or don't have my normal AF