They would only do an internal scan if they weren't sure of something or perhaps your dates were out.
I had one at 12 weeks as bubs measured 9 weeks and didn't have a heartbeat so they wanted to make sure that they hadn't made a mistake. It wasn't bad at all. I just went behind a curtain, took my bottoms off and wrapped a blanket round me. Then went out, sat on the bed and she did it. You're not on display for all the world or anything and it didn't hurt.