High hcg levels but Ultrasound says less

Visited my local ER. I have 1 child, 15 years old. A silent miscarrage and years since of no success til now. I spent the whole day in a dull pain before deciding i should go, just in case. End results, everything okay. But my nurse was a little confused. She said your levels appear that your about 3 months but the ultrasound shows 6 weeks. Anyone else heard of this... im just curious if this is common. Or maybe means something.. maybe nothing at all. When i asked about multiples, she said their only one sac but thats all they can see as of now... just left me wondering.. why would that happen..