The word 'pain' is a funny one. It's kind of like the word 'love' in that everyone's definition of it may be different, and that people might not be able to outwardly tell when you are experiencing it. There is actually a textbook definition of pain, which is caused by chemicals that are released when there is tissue or organ damage in your body. These chemicals travel to your nervous system, and they are then transmitted to your brain to send your body the signal that you are in pain. This can easily be seen as an evolutionary mechanism. If we held our hand in the fire, and it hurt, our body would very quickly transmit the message to move your hand.
When we think about pain with this definition, it makes sense why many women say that childbirth is painful. During labor, your cervix is opening, and while this is a normal and wanted action of labor, the tissues of your cervix are being pulled and stretched by the muscle of your uterus, hence causing you 'pain.' Do you ever experience pain when you exercise? Like when you go on a really good nice long run and your muscles hurt but you also feel exhilarated and happy? Our body has a built-in pain relief system of endorphin release that happens when you exercise, and when you are in labor. When you're in labor, the oxytocin flowing in your blood causes endorphins to deliver some natural pain relief. Unfortunately, synthetic oxytocin (Pitocin) has not been shown to share the ability to trigger endorphin release.
Suffering, on the other hand is different from pain. While pain is a physical sensation, suffering is psychological. When a loved one dies and you are sad, you are suffering even though you haven't physically gotten hurt. During labor, if you are scared, nervous, or don't feel supported, you may experience suffering in addition to pain. On the other hand, if you are relaxed, trusting, and do feel supported, it is likely that you will not suffer and your endorphins will kick in to help combat the physical sensations you are feeling.
One way to explain this complex relationship between your emotions and the physical sensations is called the Gate Theory. This is a concept that is taught in most doula training programs, so if it sounds interesting, ask you doula about it. It basically supposes that pain signals can be blocked from traveling up to your brain if a 'gate' along your spinal cord has input from another non-painful source.
Every woman will describe what she felt while going through labor and birth in a different way. Some women say it was excruciatingly painful, but please realize being in pain is not always a bad thing. See pain for what it is- a physiological response to stimuli. Being in pain does not necessarily mean you are suffering too.