I say there are never too few (or too many LOL) chickens to be called a flock!
I think this is completely normal behavior for a chick because mine did this too! So this probably doesn’t have to do with the “adding more chicks” scenario. They like testing put their wings (even super young!). I mean, sure it’s possible she’s a little extra stressed and wants some more space, but she should settle and be okay eventually. I would definitely try to move them to a bigger brooder if you can, though! This may help them integrate better.
How have they been acting now? Unless there’s harsh pecking or blood, they should be fine together.
I would just double-check for no other problems this chick 
could be experiencing (I’m sure you probably already know about these 
@Senorahoney but I’ll list them in case): pasty butt, no food or water, hurt or bleeding, or too cramped.
 Anyway, hope this helps! These are just suggestions! 

 I’m no expert!