come on someone answer my question!
Ultrasounds magnify things a bit, but they don't see at a cellular level. The fifth week is always an iffy and unnecessarily alarming time to have an ultrasound. (Especially this is true since the embryo could have implanted a couple of days later than normal and you would really be still in your 4th week, when nothing is visible on ultrasounds on any pregnancy). You're probably just too early for an ultrasound to see the tiny embryo forming. Not seeing anything in the ultrasound is certainly consistent with hCG being at 1500.
Try not to stress, everything sounds normal for early in the fifth week. Ultrasounds don't magnify much, certainly not to a cellular level. At this early stage, you have to just rely on hCG tests to tell you what is going on. (All that ultrasounds done this early do is scare the heck out of people. Especially this is true since the embryo could have implanted just two days later than the average and you would really be still in your 4th week, when nothing is ever seen.) Not seeing anything in the ultrasound is certainly consistent with hCG being at 1500; it all just sounds like you're really, really early.
Sorry, didn't know the first version posted (it came back to me as though it didn't) so I edited it and sent it, only to find it doubled up!
Several scenarios could be playing out here. If your dates are off by even a couple of days it may be too early to see anything. On the other hand you may have what is known as a chemical pregnancy. I would give it atleast a week and insist on another u/s to see if they see a gestational sac. The repeat bloodwork should be able to tell you if your HCG levels are doubling as they should.