Is it normal to get an erection when you put on a jock strap?
During your teenage years, when your body first gains the ability to have erections, it is normal for just about anything and everything to trigger an erection. Your brain is trying to figure out when erections are appropriate and for a number of years it will be spontaneously triggering erections at the most odd times.
The jock strap feels different than the underwear you normally wear and it is that difference that is triggering the reaction. Worse, because the space is more confined, you are very much aware of your erection. Just relax because spontaneous erections rarely last more than a few minutes and once your mind is on your particular sport, the erections will subside. Shortly, they won't happen at all. In the meantime, when you are putting on your jock strap, make sure your penis is pointing upwards so that when an erection does occur, it will go up along side your body instead of trying to poke outwards.