Empty Uterus

Went in this morning for my first appointment, dr did vaginal ultra sound and said there was nothing in my uterus. I was sent to an ultra sound tech to spend more time and detailed look and still nothing. Dr said there might be something in my right tube but couldn’t really tell. By my calculations I should be about 7 weeks - why is nothing showing up!? I’ve had numerous positive tests (most recent one was 3 days ago) and my boobs have been changing as I read they would be. I had some brown spotting for 2 weeks - no red, no clumps, no real cramps. Dr said it might be ectopic - but wouldn’t they have seen that clearly on the ultrasound?

My periods haven’t been totally regular since I got off birth control last December, LMP was 7/15. I started light bleeding for 5 days on 8/11 which I thought was my period, but then had my first BFP on 8/19. That’s too soon for it to be the 8/11 cycle, right? I just assumed that was implantation bleeding.

Anyone else experience a similar situation? Or can offer up any wisdom or advice?