Why do girls act like they like you then change their mind, then change their mind again??

I'm talking to this girl who really seems to be into me. I got her phone number the night we met and she actually goes to college with me. I text her a few days later and she runs into me at school. We have coffee later in the week and talk for 2 hours and she really seems to like me a lot. After this last weekend I text her again asking if I can talk to her and she's like, "nah I'd rather go to sleep...". Maybe I'm being premature about this but it seems like if she were still into me she'd at least let me call her, or is she just playing hard to get? Or is she likely not that into me at all? I understand that I might just be anxious since I have somewhat of a crush on this girl but it kills me to think that the game is being played. We think of ourselves as sophisticated animals and make fun of lesser animals on the National Geographic channel, watching birds do a silly dance to get the female bird, etc... but jeezzz it seems like girls make guys do the exact same thing. Thoughts?