Is it normal i crack a fat randomly

Hi, I have a circumcised penis and my knob is fat and sensitive, I might be at work, walking through the shop or driving and it rubs on my shorts, or I might have the slightest dirty thought and bam, hard on. Then it won't go away because I am thinking about it. Quiet often I'm walking through the shops and have to lean over or sit down. A. So it goes down and B. To hide it. It's embarrassing. I guess I shouldn't complain, sometimes it's just at the worst times.