Normal to have cramping, but no period?

so i've been having cramps & lack of appetite, which is usually a tell-all sign that i'm about to start my period. however, i am on the pill, & i'm already 3 placebos into my pack...i'll end up taking my 4th one tonight. is it possible that my period will come within the next few days? i'm just curious why i'm PMSing but no actual blood is coming out :/