You know I thought it was just me. I am in my thirties now, and often have flashbacks, particularly when I exercise. I think because when I was at school exercise was pure torture for me. I had severe coordination difficulties (I have a mild form of cerebral palsy) and quite severe asthma, and none of the teachers at school knew how to modify activities to suit my needs. So I got many nasty comments at school, and sometimes at the gym, I still feel very vulnerable, and almost frightened of others.
Good on you for realising and working through your feelings, and keep going with that exercise.