Hi Dr. Grief, I gave blood last Thurs and felt fine, however today when I jogged my usual route, which includes a steep hill I felt unusually tired. Naturally I was a bit out of breath, so I started walking up the hill, however I still felt out of shape . At one point I felt a huge pain in my chest ...so much so that I clutched my hands over the center of my chest hoping to push the pain away. I have jogged 4 miles/day for 45 years. Was this just from giving blood or should I be concerned?