Since you have already mentioned that it increases when you are stressed out, it could be one of many things- though I strictly can't rule out cardiac problem because any discomfort above diaphragm ,made worse by effort or anxiety
can be of cardiac origionn but the fact that it gets worse with coffee goes in favour of anxiety symptoms. if need to have some more symptoms like feeling fearful that something is going to happen, tiredness
, increased heart rate
, sweating, tremors of hands, increased frequency of urination and so on. Anxiety ,if due to some stressful event should be dealt with as such , by problem solving and progressive muscular relaxation technique.