Help with a friend?

With all due respect completely ignore the first answer. There are a few possible reasons to her behaviour. 

Her behaviour could have nothing to do with you. Sometimes when a person is going through something their behaviour changes. This is because the negative situations in their lives manifest into other emotions which can often cause people to be angrier, colder or excessive happiness or sadness this can especially happen when the person ingnores their emotions for too long.

They're generally not interested in you romantically. Sometimes it is necessary to read emotions as they are.

Some people are proud and too scared of rejection, that's why it's easier to give you the cold shoulder.

(Not very likely) she could have issues with expressing her attraction towards you or had previous bad experiences with romance. This can also may be a way of her telling you she wants to slow down.

Or it may just be her personality as some people are  naturally cold towards others.

Either way talk to her. If this girl is anything like me (a female teenager) if you ignore her, she will ignore you right back. I don't know who even decided this was a good idea but ignoring someone ussually never works in making someone like you back as they will only feel unwanted and get over you. 

I wish you all the luck x

Time will tell. These things can happen if a person seems too keen it is human nature to take them for granted.

Go out and about as much as you can so you meet different people and do not focus on this girl.

If she does contact you and you have decided that you want to be with her ask her what she wants from your relationship.

But stop the contact for now and hopefully she will wonder what you are doing and will be more keener.