Post partum Pregnancy?

I am now almost 12 weeks postpartum following delivery of my first child, a healthy son. We had no trouble conceiving, he practically had only to wink at me and Boom! We resumed intercourse at 6 wks, and had done so a couple of times since. I breastfeed exclusively, and have not gotten my period yet. However, recently I feel almost as I did when I was first pregnant-loss of appetite, nausea, exhaustion, tingling breasts. Much of this could be attributed to breasatfeeding/ being a new parent, but what are my chances of being pregnant again? And if so, how would I know? Do I still have hormones in my system from the pregnancy, or would a pregnancy test detect new pregnancy hormones?