Eila probably got her bump on the head when she leaped up in surprise and hit her head on the top bunk of the bed. It's not really shown, so it could be the result of Sanya's fist too. That's also a fun way to interpret it.
Seeing Eila faint in agony should have calmed Sanya down, yet in the following scene, she still seemed a little annoyed. Thinking about why that is could also be fun.
You should always try to interpret thing in ways that are fun, or cute. My way of taking things is just my way of taking things.
Having said that, there are some people who always take things in bad ways. They just get a consolation prize.