When you're moving furniture around, and your partner manages to ram your brother into the wall with the queen mattress (really hard), so you laugh.
Then, 10 minutes later, your brother gets your partner back by squishing him between two bookshelves (really hard) and you laugh so hard you sit on the floor and cry your eyes out!
And then later in the day you remember the "OOOFF!" your brother made when he got squished by the mattress' momentum, and you just lie on the floor and howl with laughter til the cats start avoiding you!
You also know you have PTSD when you're sitting behind the wardrobe door silently bawling your eyes out and the kitten has never seen a human cry before, so looks around the door at you with massive big scared eyes, and keeps meowing at you.