YA book about girl who gets locked in a library and finds out someone else is there

[Name]

This book was likely written around 2000-2017.
The Main Character girl is on a road trip with her friends and the guy she likes. She goes into the library to pee and gets locked in. Her friends don't notice she's in there because they are taking two separate cars and each group thinks that she took the other car.
While she's stuck at the library, she finds out that there is a guy from her school staying there. Dax or something was his name. They end up befriending each other and survive off vending machine food basically. Also they find a bunch of reward type toys and get one of those slinging hand things.
Along with that, this girl has anxiety and after she's out of the library. She finds out the boy she originally had a crush on, got into a car crash.
Etc Etc. At one point her friends jokingly kidnap her and bring her to a party or something, in which she freaks out because of her anxiety.
Finally, the boy she had a crush on makes a move on her but she doesn't like him anymore and likes the Dax guy.
Also, Dax is in the system.

Okay that was really long, but that's basically everything I remember about the book.

[Bargle]

Link for Olivia's suggestion By Your Side by Kasie West.

[Kris]

Olivia's suggestion of By Your Side by Kasie West sounds like your book.

Amazon's "Look Inside" preview:
https://www.amazon.com/Your-Side-Kasi...

Kirkus Review:
- "After a weekend trapped in a library together, two teens from different social stratospheres are drawn to one another.
All Autumn, a white senior, wanted to do was go to the bathroom. She had been studying with her friends in the library before it closed, excited to go build a campfire in the canyon. Instead, she finds herself locked in the library, her friends gone and her phone with them. But she’s not alone: Dax, a white loner with a bad reputation, has purposely stayed behind, unaware of her presence. It’s a three-day weekend, and the two while the hours away with games and standoffs, each wary of trusting the other. ..."

[Darcey]

By Your Side definitely sounds like your book. Good book!