I started having center chest pain that went down to my belly button and pelvis. the pain was stabbing and unbearable. what can cause this?

Multiple Causes: Chest pain can be a symptom of heart disease but may be may other things such as esophageal spasms, great vessel disease, respiratory disorders (asthma, COPD, bronchitis, infections), and muscle problems to name a few. It really depends on the clinical situation and other symptoms you may be having. it is very unlikely that at 18 it is serious but should see a doctor to get more information.