There have never been any reports in the medical literature of STD transmission in this manner and I cannot imagine there is significant risk. To be at risk, the previous person examined would have to have had an STD, which probabl is unlikely; and any secretions from her vagina would have to still be moist, i.e. the two exams would have to be a few minutes apart, at most; and on top of all that, the ultrasound personnel would have had to forget to clean the probe. The chance that all these unlikely events would all go the wrong way is infintessimal. Please don't worry about it. But if you can't get your mind off it, you need to pursue your concerns with your obstetrician.
Really, don't worry about this. Best wishes for an uneventful pregnancy and a healthy baby. HHH, MD
All the best,
Patty824