What Causes Short Periods With Cramping?

im on nortrel 7/7/7 a 28 day pack i finished the last yellow pill and i missed my first active pill in the pack and i had unprotected sex the next morning and 19 hours later i took the missed pill. and for some reason my period was only 3 days but it wasnt like the rest of my periods i only bleed a lil one day then the next i didnt bleed then i bleed a lil more and that was it my periods in usually 4 to five days with cramps this time i cramped but i didnt cramp could i be pregnant