Thanks to both of you for your quick responses.
When I ask her what's up, "nothing" is the general response - sometimes she will tell me if there is something up, but when there's not and she's still down, this is where I struggle.
I've considered keeping the MSN conversations shorter, but sometimes we'll be having a great time, all smiles, and then *bam* she might change. I considered it to be 'Push-Pull' and even thought about confronting her on it but decided against it and ran my own game instead - being positive always, trying push-pull myself, teasing.
I also know she loves me to give her 100% attention, she's said it

Sometimes I will if she's being nice, if not, I won't

Keep her on her toes y'know.
She wants control though, and the fact she has always had a BF for as long as I know whereas I have little experience, means her natural game is much stronger and I can feel myself working very hard to stay attractive.
I do not want her to become bored with me, I kinda like her, and she has said the three magic words (ILY), but she's still testing me.