Sometimes i get a tightness in chest / top of abdomen. went to hospital, heart is fine they said. what else could 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), muscle problems, or anxiety to name a few. It really depends on the clinical situation and other symptoms you may be having. At your age it is unlikely serious but should see a doctor if things persist.