400 HCG at 6 weeks?

Hi ladies.. I went to the ER for my back pain and I’m supposed to be 6 weeks today. The back pain was most likely a pulled muscle but they did an ultrasound to be sure it wasn’t an ectopic pregnancy. The ultrasound didn’t show anything so they sent me up for a transvaginal ultrasound and she still couldn’t see anything not even a gestational sac... she said my HCG levels were at 400. Could I be miscarrying? I’ve had a miscarriage before and I honestly don’t Think I can mentally handle another it put me in a bad place last time... they said I’ll go back to OB on Monday or Tuesday to get blood work again and check to see if levels are rising or not.. I’m really afraid.. anyone else have this happen to them?