I think you should insist on a follow-up ultrasound within a week.
Honestly, the dates don't add up. If you got your BFP in December 19th, You should be 8 weeks or so. You wouldn't have been able to have a positive pregnancy test 4-5 weeks ago and still only be measuring 5+ weeks. There are some stories where things turn out ok, ie, the baby just is really slow growing and measuring behind, but these are rare stories.
I've been there, and limbo is a really really horrible place to be in. I really hate saying this, but I think you should prepare yourself for not great news.