Why does she act so distant when we are apart?

I have been seeing this girl for nearly a year now and although we live in different cities we get to see one another a few times a week. When we are together all is great and we get on so well, we often talk about a long future together.

Problem is this.. When we are apart a lot of days she barely texts/calls. When we do talk she doesn't seem as invested as when we are together and the conversations are short, dull and it feels like a task for her. When I have questioned her about this she says that she is tired and needs to sleep etc. So It's really bothering me and is it a sign that she doesn't want me anymore?
Why does she act so distant when we are apart?
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