why am i so afraid of clowns?

[Hello]

i am a 14 year old and i freak out when i even think about thinking about them i can't be near one without running the other way or starting to hyperventilate all my friends know this cause i freaked out at my best friends b-day party WHEN WE WHERE 12!!! and they all think it's funny one of my friends gave me a pic of a clown for my birthday and i seriously don't know why i am afraid of clowns i have never had an experience with a clown except for my friends party and i have been scared of clowns sense i was very little but i don't know why so i was hoping a few people outside of my group of "friends" would give me some insight as into why i have recurring nightmares and paranoia of clowns?!