Boy or girl?

So I went to the doctor yesterday at 16 weeks. I have a cyst on my ovary that they wanted to check so they did an ultrasound. She also looked for baby gender. She said boy! 100% Compared them to those of my 2 year old son (taken at 20 weeks) and am not convinced! Could she have been wrong? She looked for like 2 seconds... I had not prepared myself to find out yesterday and was a little annoyed that she said 100% when it is 16 weeks and only looked so quick. What do u all think?