The uterus muscles are smooth and have few pain receptors. Your autonomous nervous system controls your uterus muscles. You cannot consciously control these muscles with your physical strength or will. When you feel pain during a contraction you are primarily feeling the stretching of neighboring muscles, which have pain receptors.
If you totally relax, especially your abdomen and breathe deeply, your uterus will contract with minimal resistance from you, which means less pain.
This is very important to remember: you cannot stop the contractions of your uterus. The more relaxed you are the better the muscles can perform their function. If you are tense, you work against the function of the muscles, which causes pain and tension. To assist your body to stretch and expand, your body secretes the hormone, Relaxin, which softens muscles, ligaments and tissues throughout your body.