The best thing to do is to call you dr. and make an appt. and then he will have to set you up with an u/s. And yes, it only takes once.....
Yes it could just be that one time, I miscarried Feb 7th and got a BFP March 7th and I could just about tell you the day it happened....
I forgot, my Doctor told me to consider the first day of my miscarriage the day first day of my last period so...also I dont think your levels will go all the way back down if you miscarry 1 and are still carrying the other.