Have Anxiety And Panic Attacks. EKG And Ultrasound Normal. Cause?

Hi, I m a cardiophobe. I suffer from anxiety and constant panic attacks. I just had a spasm over my left breast that almost sent me into a panic attack and I thought it was the start of a heart attack . I m 21 years old, I ve had several ekgs done, several blood test , stress test, thyroid test, heart ultrasound, and all have came out great. Doctors have told me the feelings I get in my chest are not heart related but yet they scare me. I need some reassurance and want to know what can be cause these chest spasms over the heart area. Thank you.