Hi, was it an internal or external ultrasound?A large mass could be fibroids,polyps ,both of which are not unusual. I would not worry about the speed,it's normal,because they want to rule out anything serious quickly. I recently had internal and external scan,then literally days later I had a coloscopy,biopsy and had a fibroid removed. It was very fast and I'm waiting for biopsy results,although everything looked ok except for fibroids.Its really quite frightening when you don't know what it is and the medical terms don't help. Try not to worry.

On the outside of tummy. 

Ok. Don't worry. (Easy to say I know) its normal procedure.It was your doctor that did external but unable to do internal,which would more likely show what the mass is. They fastrack you to gyno so they can ensure early detection of anything untoward. They are not convinced it's cancer,they just want to rule it out quickly.Gyno can do an internal scan,and coloscopy if required. I already knew I had fibroids because I had an internal scan 4 years ago,so it was not so scary. Whatever it is ,know that it's treatable.Hope all goes well.