I Have Been Having Squeezing Pain Right Under My Left Breast For Several Weeks Now On And Off. What Could Be It Is?

I have been having this squeezing pain right under my left breast for several weeks now on and off. Recently I developed a migraine and was ill for 2 days. The second day I had a feeling of fullness in my chest and upper back and general malaise. I went to my doctor who did an EKG . She didn't like the first result, I guess, because she did another one after giving me sublingual Nitro and an aspirin . We waited 5 minutes and while I was undergoing the 2nd EKG, I felt like I was fainting and my skin was cold and clammy, but I was also hot. They noticed some abnormalities in both my EKGs and admitted me to the hospital. All of my bloodwork came back negative in 24 hour period. I was given 2 types of steroids during my stay and also Darvocet and Xanax for my severe headache . Then I all of a sudden felt better. They took me to do a stress test an hour and a half later. I passed it. So my question is why would my EKG's at the doctors office be abnormal enough to send me to the hospital, but a stress test was negative the next day? I have a followup next week with my doctor, but I was having a heart attack as far as he knew. Could the drugs they gave me in the hospital have helped me with a good stress test reading?