Ultrasound but no baby?!

So I’ve been having some pretty bad cramps so I ended up going to a emergency room. I’m 4 weeks and 6 days and she said she saw no evidence of a sac or anything at all in fact. My levels were at 700 so she said i might just be to early or something else like a ectopic. Has anyone else experienced this? Please tell me any information you have, good or bad :) also, no blood at all, just the cramps