OP here - I agree with the most recent PP that I just have to wait it out until my next scan in 2 weeks. I did call my nurse again though, and she said not to worry since the spotting is brown and minimal (i.e. not an active bleed). She also said spotting is a little more common in IVF pregnancies and could have been caused by the US. Time will tell, but I still have nausea, so that's a good sign at least. Thanks, everyone.