HELP when did I conceive !

Hello I’m new to this group I just have a question. So I have irregular periods I think I had a very short period in feb where it came on went off then I was just spotting but I wasn’t pregnant . Okay so in the end of feb I had intercourse feb 18th and feb 20th . Not sure if I had a slip up or not . So time goes by I had my regular 7day cycle on March 3rd and had intercourse on the 11th and the 12th . I tested positive two weeks later but had a doc apt on April 2nd which showed I was pregnant but ultrasound wasn’t really showing anything. I have a due date of december 8th . Could I have gotten pregnant from the days I had intercourse in late feb or is this After my cycle from March !