Is chest pain during exercise normal?

Yes, your workouts should feel challenging, but chest pain during a workout is never normal. If you are experiencing chest pain during a workout, stop, let someone know, and call 911.

What hurts when doing sit ups?

Since the hip flexors attach to the front of the lumbar spine, if they become overworked or overtightened they can create an anterior (forward) pull on the lumbar vertebrae, which might be the reason for the discomfort.