Random pain that is not caused by any known factor and that is significant enough to make you cry is not something that is normal, and does need to be discussed with a physician soon. The vagueness of the remainder of your question precludes the ability to discuss the problem in more detail, but some common causes of pain will be listed.
Trauma or a history of trauma can obviously result in pain, and that goes for a history of surgeries or other known medical problems that you might have or have had in your past. Heart problems can be the cause of chest pain in many people, although it is less common in 18 year old people. Medical conditions that are poorly treated can also cause some kinds of pain, including uncontrolled diabetes. Usually these will have other symptoms as well, such as weight loss
, increased urination, changes in your energy level, or other similar complaints.
There are other causes of pain as well, such as the pain that accompanies serious medical problems that need to be treated immediately, such as unregulated cell growth. Please discuss your symptoms in more detail with your primary care doctor
immediately, and know that spontaneous pain without a known cause can be a serious symptom that you should not delay in having evaluated.