If you had a comprehensive workup done three months ago and everything was clear AND you're only 21 years old, you are in the best possible situation to not worry about a heart attack.
Anxiety about a heart condition can absolutely cause you to experience tightness in your chest. You know how if you have an injury you tend to tense up your muscles around it in an effort to protect it? Or if someone goes to tickle you somewhere the muscles get all tense around that area? This is a reflexive action that you don't even consciously do.
In this exact same way, when you have anxiety about your heart, you tend to tense up your chest muscles in anticipation of it needing protection. This tension ironically creates the very symptoms you dread which makes you think you are having a heart attack. You need to work on relaxing those muscles and not stressing out about your heart. You have no good reason to worry about it.