I'd ask them to do either a CT or MRI on the next go around. No sense in doing another US since the mass detection seems to be at the limit off that technology. Both are better suited in characterizing the mass anyway.
Is it possible for a 2 cm mass to "disappear" within 2 mths?
It's certainly possible for a small mass to just disappear, especially if it's comprised of focal fat or edema. But your doctors haven't been able to determine what is present at this point. If it was me, I'd like them to move forward towards identifying what's up using the best imaging modality available. At least for me anyway, it would give me peace of mind. And we're not doctors here so speculation isn't of much use.