No Baby in ultrasound

I had an ultrasound yesterday at the er and they still could not find a sac in my uterus but ruled out ectopic pregnancy so far ! They have been monitoring my hcg levels for a couple weeks and they rose to over a thousand ! The dr said I was right at the cusp of when you should be able to pick something up on the ultrasound but that I should have my obgyn do a follow up ! My cycles are irregular so idk if that could play a part but has anyone not seen a baby on the ultrasound yet with rising hcg and everything be ok ? I’ve heard usually you have to have 1500 to 2000 to see anything ! This is my 4th and I’ve never had these issues