How Early Can Pregnancy Be Detected After Having Unprotected Sex?

my cycle for this june was unusual. i was due on the 25th and i started cramping then for three days i was only spotting. i ovulated between june 9th and the 11th. i had sex at that time and used the pull out method. my periods are always heavy in flow and red in color. i havent been stressed at all. i have taken three pregnancy tests and they were negative. can i be pregnant and too early to tell?