Unexpected Pregnancy Test: Late Period & Positive Result - Seeking Advice

Question
HI THERE, IM 23 YEARS OLD, I HAVE A 4 YEAR OLD SON AND A 7MONTH OLD DAUGHER,MY HUSBAND AND I  HAVE ONLY BEEN AVOIDING SEX DURING OVULATION (ACTUALLY ABOUT 4 DAYS BEFORE 4 DAYS AFTER) TO PREVENT PREGNANCY.  MY LAST PERIOD WAS FEB/10/2005, ON FRIDAY MARCH 18/2005 THERE WAS NO PERIOD SO WE TOOK 3 TESTS ALL WHICH CAME OUT POSITIVE, VERY FAST AND VERY DARK LINES. I WAS SHOCKED ONLY BECAUSE WE ONLY HAD SEX ONCE THIS MONTH AND IT WAS MARCH 1ST OR 2ND. I DIDNT THINK THIS WAS POSSIBLE?WITH BOTH OTHER PREGNANCIES OVULATION/CONCIEVING ADDED UP CORRECTLY. ALSO, I GOT MY LAST PERIOD (FEB 10/05) WHEN EXPECTED BUT IT WASNT AS HEAVY AS NORMAL, THE LAST PERIOD BEFORE THAT WAS JAN/10/05 AND WE DID HAVE SEX PRETTY CLOSE TO OVULATION DATE, IS IT POSSIBLE THAT WE GOT PREGNANT THEN AND I EXPERIENCED "IMPLANTATION BLEEDING", OR DID I CONCIEVE THAT ONE TIME WE HAD SEX A WEEK AFTER OVULATION? WE ARE BOTH HAPPY, JUST VERY CONFUSED!?! PLEASE LET ME KNOW WHAT YOU THINK. THANKS VERY MUCH!
SINCERELY,
AMBER.

Answer
From your description, I cannot figure out exactly when you conceived. Certainly, if your last period started on February 10, then the earliest that you may have conceived would have been on February 23. A sonogram will further pinpoint the day of conception.