No sac 4w5d

According to my lmp, I should be 4w5d but I didn’t get a positive opk till about cd16. I had some brown spotting this morning so my midwife sent me for a sonogram. They did an abdominal sonogram and didn’t see a sac - the tech said normally they’d see something by now. They took my blood so I should know more tomorrow when I get results, but I’m really scared I’m gonna miscarry. Does anyone have any experience with anything like this?