Could i possibly have conceived the day after my period?

My boyfriend and I had unprotected sex the day after my period (the 23rd) and on the 26th i experienced reddish-brown spotting (when i sat on the toilet and peed, what looked like a small clot of blood the size of a dime basically fell out of me). The spotting seems to have stopped today.
I am extremely tired and dizzy upon basically any movement, my headaches are never-ending and excruciatingly painful, i have occasional stomach cramps, frequent urination and yesterday i ate a whole rack of ribs and was still hungry!!! I also just recently went off the birth control pill - had my period - then had unprotected sex. Is it possible I'm pregnant and how soon can i take a pregnancy test?

You can get pregnant at any time, I would wait a week and then take a test.