I Took 5 Pregnancy Tests, All Different Brands And They

I took 5 pregnancy tests, all different brands and they were all positive in the past 5 days. I would only be about 3 to 4 weeks pregnant at this point. I had very light bleeding only when I wiped so I went to the ER. They did a urine test, positive result, They took my blood to test my hcg levels which were at 81 and did an internal ultrasound and couldn't see anything and my cervix was closed. They told me they didn't know what was going on but I was probably having a failed pregnancy. I thought you couldn't see anything on an ultrasound that early and that levels of hcg varied that early on. She I be worried or could the ER have been wrong?