It is possible to be pregnant soon after a miscarriage. It would be too early to know one way or the other by ultrasound. I suggest you call your OB and say your beta hCG on the 5th was 400, and you would like to have a follow-up beta hCG to see if the number is going up. It would be a logical request given the situation. If you are withholding your medically related information or requests for appropriate care from your doctor because you don't want him to get mad at you, then you need to find a different doctor -- an ER is no substitute for a regular medical professional who knows you.
im sorry for your loss. It could be possible that your pregnant but it would be too early to tell by ultrasound. I miscarried march this year at 11 weeks and only bled for 7 days when i had my follow up i was told to do a pregnancy test (thinking it was strange as i had just miscarried) i took the test and it was showing that i was pregnant i was then sent for a scan nothing showed on the scan. so i was booked in with the doc 3 weeks later for another test, i went back done the test and it showed negative. I was then told by the nurse that it can take upto 6weeks after a miscarriage for the pregnancy hormone to settle down and stop giving false positives. good luck.
It is possible to be preggo but it's also possible your levels just aren't down yet. Mine took 9 weeks after a D&C to be back to below 5. It was very very frustrating. My doc. took my levels once a week to make sure they were going down. Good luck