It seems to me far too early the date the ultrasound was performed. First, after sexual contact, fertilization can occur within the next 24 hours, and after fertilization, it lasts approximately 7-10 days until the newly formed egg attaches to the uterus, it is removed.
Only after the time of nesting is secreted HCG (pregnancy hormone), that hormone that can be identified in urine or serum to define pregnancy diagnosis.
Ultrasound is usually recommended at 6 weeks of amenorrhea, before this period it is unlikely that an image can be identified to certify the presence of pregnancy, and the absence of the image at such a distance from the possible fecund sexual contact cannot exclude the diagnosis of pregnancy .
As a result, I recommend that you perform a pregnancy test 10 days after the sexual contact and repeat the ultrasound at least 4 weeks before this contact.