Devastated low hcg no sac found on trans vaginal ultrasound

So I had a full cycle in January (jan22-26) February (February 18-20) I had what I thought was a super light cycle but had to be implantation bleeding. Took a test just out of curiosity on February 20 of course it was negative. Has mild brown discharge on March 3 took a pregnancy test BFP! This morning March 8 has mild cramping with a little bit of brown discharge. Panicked and went to the ER. They took a test positive than blood work my hcg levels were extremely low 48 they said I should be almost 6 week and my levels should be in the 1000s plus and that I should prepare for the worse. They did a trans vaginal ultrasound no sac visible. But no ectopic pregnancy either. They want to retest my hcg levels on March 20,2020. I’m so heartbroken and devastated. 😭😭😭pray for me ladies.