Hi I Have A Question I Had Sex Wit Me

Hi i have a question i had sex wit me ex on the 5 september but i had a period on the 16-18 the first day was heavy the other 2 days was not so heavy the rest of days me an my boyfriend had sex my early ultrasound said i was 5weks and 5days .who is the father but now im reading about implantion bleeding im confuse pls help me