Generally, once you become pregnant your periods will stop. I have been pregnant five times (two miscarriages and three pregnancies) and knew pretty well that I was pregnant after a few days. I've never had a period after conception. There are stories of women having a period while believing they were pregnant, but maybe they weren't actually pregnant then and became pregnant after the period was over.
Just for your information about becoming pregnant, from what I know and my experience, there is a only small window of time when a woman can actually get pregnant - it is between the 9th and 14th day from the LAST day of your period.
As far as my pregnancies, I know what day each of my children were conceived which gave me a near exact due date and they were all born on their due dates. My first son was conceived July 18 and he was born April 15. The second was conceived on Valentine's Day and was born November 10th. My twins were conceived on March 15th and were born on December 13th (amazingly, they were full-term which is not usually the case with twins).
I hope that helps. You probably aren't pregnant if you had a period. If you have a Dollar Tree or 99 Cents Store near you, the pregnancy tests they sell there do actually work. Just FYI.
Have a great day.