Strange dropping sensation in chest, makes breathing short and heart fast. Ct scan clear, chest xray clear, not pulmonary embolism or dvt. What is it?

Event recorder. Supraventricular tachycardias are fairly common in young adults. This can cause similar symptoms. To know if that is the case, you need to have an event recorder to determine what is your heart rythm when you are having the symptoms. Transient atrial flutter or other supraventricular tachycardia with rapid ventricular response could cause it. Cut caffeine, nicotine and stress. Beta blocker.
Arrhythmia. You sound like you have heart rhythm issue. A 24 hour recording of your heart rhythm (holter recording) would be helpful.
Mitral valve prolaps. See Cardiologist for 2D cardiac echo. Atenolol may help but need to make diagnosis first May also be panic attack ?????