Often when I do cardio exercise I feel a sense of well-being but then a couple of hours later I start to have feelings of panic. I assume the neurotransmitters that make me feel well-being begin to crash after a few hours and then I feel the exact opposite of well-being. This seems to be more pronounced when I exercise more intensely.

Just wondering if others experience this? It would be really scary except that I kind of know what to expect now and I can talk myself through it. I try not to exercise so intensely that it happens, but it doesn't take much.