hcg after miscarriage

I had my second miscarriage at 6 weeks last Thursday  the 16th went to the er and my hcg levels were low at 414, they did an ultrasound and pelvic exam and said they couldn't see anything so they said it was most likely a miscarriage since I was having very heavy bleeding and cramping and my hcg has now gone up to 1,090. I've been having vaginal ultrasounds but the doctor says they can't see anything anywhere so I have to keep going in for blood tests. They say there's a chance it could be an ectopic pregnancy but it's still too small to see or there is remaining tissue. But if it was any remaining tissue wouldn't they be able to see it on an ultrasound? Is this normal? Has this happened to anyone before? My last miscarriage my hcg started dropping right away I didn't have any problems with anything so maybe I'm just over reacting?...... (also, I am still currently bleeding since I went to the er Thursday not as heavy but it's more like a light period)