sorry for the long post

jamiee✨💙 • Kaiden Blake 22.06.17💙 Baby #2 due 22.06.18 ♥️
So I've had some left sided abdominal pain and have been to hospital over the last week, to have HCG and scans, HCG has come back as: 
20/10 - 3373 
22/10 - 4670
23/10 - 5147
25/10 - 6204
So not doubling at all, going up roughly 1000 every 48 hours, today I had a scan and I should be 6 weeks pregnant exactly, today the sonographer said she could see the sac, a yolk sac but no foetus yet, and the sac was measuring 12mm...which is apparently small for 6 weeks, made me more worried than I was at the start. Can anyone shed some light for me please?