Why does this girl go through phases of ignoring me?

So me and this girl (Im a girl too) Told each other we liked each other but neither one of us are gay. Well long story short we would be together if she wasn't already dating this guy so she said. Well she IMs me telling me shes in love with me and loves me blah blah blah. Then we talk about our day and ask each other how one anothers day went. We will do that for like three days in a row then she won't IM me for like a few days. Then it will start all over.