My Lmp Was August 8, My Husband And I Did

my Lmp was August 8, my husband and I did the deed last august 18, then i take 4 tables of nordette on august 19 around 9 in the morning then take the next doze (4 tablets) after 12 hours. After that, we did the deed again last night, when i woke up this morning i was surprise that I have my withdrawal bleeding? May i know why this happening now? Am I pregnant?