What Causes Pain Behind The Xiphoid Process And Dizziness?

Hi, I m a 61 y.o. female. I have a history of Hypohomocystenemia with related hyperlipidemia. I m on Metoprolol 6.25 mg BID, Lipitor 40 mg. QD. ASA 81 mg. QD. Vit B12 1000 mcg sublingual QD, Vit D3 4000 iu QD. 5 days ago I had an episode of sudden acute pain behind the xyphoid process which extended bilaterally and through to the back, with some referred pain up into the posterior throat and jaw. Thought at first it was some sort of muscle spasm and tried to breathe through it. After about 5 minutes, took .4 mg Nitro tab sublingual, waited 5 minutes with no change in pain level but became very dizzy and co-workers activated EMS. I was transported by ambulance to ED and admitted for 23-hr. observation. Troponin tests were negative, D-Dimer negative, Cardiac Stress test negative, CXR negative and Cardiac ultra-sound(Echocardiogram) negative. This happened 1 day after the 1-yr anniversary of my husband s funeral. I was not feeling particular anxious at the time. Not sure what to make of this, but given my reaction to the Nitro, not sure what to do if this happens again in future.