I just went running this afternoon (as per usual on Saturdays), and now I have returned home and I ve got this weird, hollow spasm-ish feeling in my upper chest. It s not right where my heart is (no irregular beats, I don t think), it s more like it s right at the top of my chest. where my collar bone meets my chest plate. It doesn t really hurt, it s just unsettling. What might this be?