Be me as teen. No older than 18
Take a shower
Walk in the hallway unaware that there are people in the living room seeing you walk in the hallway with a towel on only
Dad comes in the room when you are about to dress and he's very pissed because the people in the living room saw you walking in the hallway with a towel on only. You still have the towel on
He starts yelling at you
Something strange happens
Dad changes into something non human
His eyes turn from normal human brown eyes to green snake eyes and then yellow snake eyes
His skin changes into greenish black scales
His forehead grows larger with a bump appearing above his head and his face elongates into a snout like that of a lizard
All his teeth grow sharper with fangs showing up in the teeth above as well as below. The fangs in the teeth above being bigger in size
His fingernails grow into black claws that appear to be two inches long at most
During the time when all this is happening you feel paralysed by what you can only describe as some sort of telepathic force hijacking your brain. You don't feel any fear and you can't move. You can only stay still, breathe and watch the whole thing
All of a sudden dad appears normal and calm
The feeling of being paralysed is gone
He looks at you with a slight regret on his face and leaves the room without saying anything
You are in the room by yourself wondering what the heck you saw and if you where hallucinating the whole thing
You don't tell anybody about it because people will call you insane
You definitely don't talk with dad about it
At some point in your early twenties you do a brain check and you turn out to be just fine
What happened that day never happened again
Life goes on normally

your dad probably has a prominent reptilian aspect and what you saw is part of the shapeshifter phenomenon people are always talking about. If it only happened once then he most likely has no idea and you just more "inclined" the notice these type of thing