I have this random chest pain that will come and go every now and then. It happens after being a bit down, but it tends to be extremely painful. Wory?

Probably not. At 21, random chest pain is likely musculoskeletal. Mention it to your primary physician, and hopefully you will be able to be reassured.
Need H&P. The etiology of the chest pain is very broad. Given that it is associated with mood makes me concerned about a psychosomatic pathology or an anxiety/panic disorder. That said, these are diagnoses of exclusion because your physician will likely want to make sure your heart is not the problem by doing a history, physical & poss tests. Another common etiology would be musculoskeletal dz. See your pcp.