Concerned 😢 help appreciated!

So according to glow I am 4 weeks & 4 days. I have been cramping on one side for about a week and my doctor recommended I go to ER this weekend to check for ectopic pregnancy (I've had no spotting or bleeding). My only symptoms are fatigue and frequent urination. They did transvaginal ultrasound and were unable to see anything at all, no sac, nothing in the tube, no cysts. My hcg level is 600. Anyone experience this? Getting blood drawn tomorrow to compare but as of now doctor thinks I will miscarry since no baby was seen on ultrasound and low hcg levels 😭. Anyone not be able to see anything on that early of an ultrasound? My dates should be pretty accurate as I used ovulation kits etc. thank you, feeling so sad!!