TER General Board
The same thing happens to me during my workouts. Whether summer or winter, my nipples are clearly visible even through a powerfully strong running bra and LuLu top. Mine are extremely sensitive and not to share TMI but Body Balm helps the chafing during marathons/long runs/bootcamp workouts.
Maybe we can workout together some day and stir up a crowd? 
Extremely sensitive?....
Hmmmm.....! 
It's not about blood flow going to your nipple bc that's not exactly how the human body works. It's more about the fact that you are using your pectoral/chest muscles in some way. Whether its doing arms or just moving your arms on the treadmill, etc. The muscles are straining, even slightly, which makes them bulge, and tell the nipples to get hard and contract to prevent any damage. Sort of an innate human characteristic. Any form of physical activity will make nipples on men and women contract normally.
Other than extra clothing, just rock them proudly