my hcg levels are not doubling and scan showed

hi i was wondering if anyone else was having or had this problem, i havent had a period since 25th july 2010, on the 30th august i had a positive pregnancy test, on the 4th i woke to brownish spotting no blood and it wasnt alot. On the 11 th september i went for a scan and it showed a sac but nothing inside. they said i couldve had a implantation bleed, and i was too early to be able to see anything. they checked my bloods and in 48 hours they only went up by 59%. i went again yesterday for another scan and the scan had grown but was still nothing in it. They did bloods and i phoned for results this morning and they havent gone up a big anount they are 3142, i am going out of my mind with worry, i am going next monday for another scan to see if anything has changed, i really hope it has. the doctor says they are not ruling eptopic but has the sac has grown my levels are rising and because i am not suffering with any pain they will leave me and see what happens. please has anyone else been through this