Empty sac?

Anyone else have similar problem... According to lmp I'm just about 7 weeks and yesterday I started spotting and cramping/back pain. Spotting mostly brown and pink some red here and there. Today it felt like my period was starting and there was more spotting.  ER dr did hcg and an us which I measures 5w5d and there was only a sac. Hcg level was 9000 so dr thinks I am just really early and wants me to follow up with OB. Anyone else have this happen and have a good outcome on your follow up?