Dropping hcg levels

At 3 weeks 3 days hcg levels was 14

3 weeks 6 days 203

4 weeks 1 day 637

4 weeks 4 days 2,077

4 weeks 6 days my hcg levels has dropped to 1,337.. has anyone experienced this?? There is a gestational sac on my ultrasound. I am currently 5 weeks today.. my doctor was so surprised how high my levels was moving up ..ended up at the er for being super nauseous and having a little bit of back pain, the er said it's a threatening miscarriage.. I dunno what to think or feel !!