What Does Tightness In Chest With Difficulty In Breathing Suggest?

I had tightness in chest hard time breathing. Ekg was different from 4 years ago was hospitalized over night nitroglycerin pills stopped th pain had another attack was given more nitroglycerin and I bottomed out. Had the vein thing done today no blockages to my heart. I have allergy I take shots for them. Going to do ultra sound on Thurs of the heart ..do you have any ideas.