Tightness in chest, difficulty breathing, but heart rate's normal, what could it be?

Heart problem: First and foremost hear problem should be considered and ruled out . Lung problems including asthma and chronic obstructive diseases also may present with tightness of the chest and shortness of breathe . Clot in the lung can have similar presentation. After all the life threatening medical conditions are excluded , anxiety disorder can be considered . I suggest you go to your pcp , ed and checked.

Many causes: Could be cardiac origin- blockage in arteries, or valve problem; could be lung origin- asthma, emphysema, pneumonia, bronchitis; could be unrelated to heart or lungs- anxiety, anemia, etc. Need a good evaluation to figure out what testing to best evaluate this.