Is it normal i get the urge to bite my toes while watching porn?

Lately I've been watching porn more often and everytime I do I get the urge to start biting my toes.. and I have to say it feels great.. and this happenes everytime. it started 5months ago and now I've started biting my toes while masterbating and I think it is amazing.. wwhat makes this happen to me?
P.s I do gymnastics. I'm a 18 year old boy.
Thanks