Well, I have experienced what you are saying and I was pregnant! The spotting I experienced was implantation bleeding and the cramping was from the changes in the uterus.. my sister is also one of the fortunate ones who only has a 4 day period and she also experienced spotting for one day when she found out she was pregnant. So, my best advice is take a pregnancy test and go from there. If it's negative I would wait another week and take another one.
Good Luck. Keep us posted.