X.O.
I say it is normal. My 5 yr old still does this from time to time, although now it is only when his shirt is off. He used to do it often, regardless of who was in the room and what he was wearing. He gets sensory input from it--cause & effect: if I pull this, I will feel that... Eventually he'll stop doing it.
No worries, Mama, and no need to be embarrassed of ANYTHING when you go to the doctor. My husband is a doctor, and believe me, there ain't NOTHING that is going to shock a pediatrician :)