gelp

Hi everyone my name is Jammie and I am in need of some help. I found out on the 25th of April 2009 that i was prego with an HCG level of 5(blood test). Since then weird things have been happening. My Hcg levels have not doubled like the dr. thinks they should be. On the 5 they were 90 on the 8 they were 140 and then the 240 yesterday. Last night I was told again at the hosptial that my hcgs are still very low now at 240 what should they be. I have had three u/s and last nights was the only one that has shown anything. It showed a 2mm sac. I am not sure if its just a pusedo sac that can show up with an etopic or if I should wait and see if r  lil one develops. I have an appointment on Tuesday to rerun my hcg and do anyother ultrasound. I do not know what to do or what to think. I have had a previous preg. and all was fine.  whats the difference between a psuedo sac and a sac that will eventualy have a baby in it.