Confused!!

Hi All

On 25th Nov I had a positive pregnancy test, and calculated I must be 4 weeks pregnant. About 10 days later started bleeding quite heavily and went to EPU who couldn’t find anything and were concerned ectopic (bloods showed HCG 50). Been back for weekly scans and bloods. HCG slowly rising but scan was clear until last week they saw empty pregnancy sac. Today’s scan shows embryo with a heartbeat but only 4mm - and it should be around 40mm by now. I am totally shocked as was expecting to have Methotrexate today... however I know as it’s so small it’s chances of survival are slim. Anyone experienced anything similar with either good or bad outcomes?