Hi, I am also 26 and have been experiencing pain to the right side of my left breat. Its like a sharp stabbing pain that comes and goes and when it comes it only lasts about 3 seconds and I usually have to stop what im doing cos its that sore! Just back from the gp and she said I have Costochondritis (Tietzes Syndrome) - inflamation to the cartlidge in my breast bone. Go get it checked!
If you are worrying about your heart, you will get chest pains. Stress causes all kinds of problems in the body and you are subjecting your body to long episodes of stress. I would just have a Doctor check my blood pressure, listen to my heart and if all is ok, I would go home and chill.
I am 30, I have same pain for over 4 months, I have been going to doctors no ender times, they all say its lak of sleep and anxity, nothing to worry about, you too young to have heart problems, but go to your GP just for your peace of mind.