Which horror movie?

Obsession.

She had a vasovagal response which is pretty common in teens.

"A vasovagal episode (or vasovagal syncope) is the most common cause of fainting. It occurs when your body overreacts to a trigger, causing a sudden drop in heart rate and blood pressure. This reduces blood flow to your brain, leading to brief, temporary unconsciousness.

Common Triggers
Episodes are typically set off by certain physical or emotional stressors.
Frequent triggers include:

Prolonged standing
Heat exposure or being in a stuffy room
The sight of blood or having blood drawn
Sudden fear, severe pain, or emotional distress
Straining (such as during a bowel movement or coughing)"

She's got four staples in her head now, and she found that process to be upsetting.

Oh, poor noggin'! Yikes on the staples. I wouldn't be pleased either.

I fainted a few times as a teen. I always knew when it was going to happen so mostly just sat down woozy. I only remember 2 clearly. One was on a first date waiting in line for a movie. I told the dude a couple times I needed to sit because I was going to faint, but he didn't believe me. That relationship didn't last. Then I told a gym teacher in no uncertain terms that if she made me do a handstand I would faint, and yeah, they took me out of the gym on a stretcher. Didn't really want to be right, and yet.