You know, something similar like that happened to me! Well, not exactly like that, but I wish.

I was having a normal check-up down there, and while my doctor was looking at my dick, he started to flip it around. At first I didn't think anything of it until, after a minute, I started to get a boner. What was hot about it though was that his face was right in front of my crotch! (He was sitting in on a stool in front of me while I was standing)

I am good at controlling an erection, but it seemed he was just playing with it! Finally, after I got hard he told me he was done...

and that was that! Could it have just been a procedure? He had to get it hard to look for something in particular? (lol)