OB dr wants an ultrasound?

So a little background;
Went to ER for itching and heavy discharge that wouldn't let up. ER dr told me I had BV. He told me to do a follow up with an OB. Made an appointment and saw the OB about two weeks later. OB did a Pap smear and swab cultures. Also did a vaginal ultrasound and found nothing. Came back negative including BV. Just talked to the receptionist and she told me that all tests were negative and the OB wants to do a follow up. She said that he wants to do an ultrasound on the last day of my next period. So call on the first day of the next period. What could that mean?? Is it cancer? Of course I'm freaking myself out. Over analyzing and over thinking.