Doctors use the date of your LMP to determine how far along you are because most women know when that was. Yes, you likely got pregnant about 2 weeks later.
The hCG levels dropping is not a good sign but there is the chance of a bad test. Are they planning on repeating it? They should be seeing a heartbeat now, or very shortly.
You are doing all that you can do. There is always hope.
Hang in there.