Hcg doubled but doc still concerned.

My periods have never been regular so I'm not sure really how far I am. August 24th I got a negative test then September 1st I got a positive then went to the docs on Wednesday 6th and said my hcg levels were 477 and then today I had some cramping and went to the docs and they did an ultrasound and only seen sac nothing else and my hcg levels was 1075, they said it could mean I'm early or a early miscarriage but I'm confused my levels are going up and we technically don't know how far I am so idk really stressed.