Drop really is like coming down off drugs, just ones produced inside your body. Physical exertion, pain, and sex all make your body release hormones and neurotransmitters that help your body deal with the exertion/pain and reward your brain by making you feel good. The ones I remember off the top of my head are epinephrine, endorphins, and oxytocin. It's very similar to "runners high". I am not a doctor or other Profesional, so take this all with a grain of salt.

In order to get your body chemistry back to equilibrium, or as an aftereffect of having your system flooded, you might feel the opposite for a while. If it was an especially physical scene, you're also going to be feeling soreness and aches you didn't notice while the endorphins did their job of blocking those sensations out.

In addition, Oxytocin helps people feel emotionally intimate with, and bonded to the people they have physical intimacy with. This one explains why a lot of people want to cuddle or have pillow talk after sex.

One of the functions of aftercare is to help people come down off of this physical/mental/emotional high.