I went on a first date on Friday, we got on well for 2 hours and when I dropped her off she kissed me on both cheeks and when I said "would you like to go out again some time" she said "we must"
On Sunday I called her asking her out on a second date leaving an answer phone message and a she texted back saying "I will let you know about Saturday later in the week". Is this no or does she want to think about it for a few days or maybe she doesn't want to appear too keen! I think she has changed her mind?

I think you're just excited and worried about rejection. When I get like this I give it a three strikes policy. If her leaving you hanging feels like she's not taking this seriously, you could call it a first strike, if you feel like that.. If she doesn't respond until you call Friday night and she says she forgot, strike two.. If she fails to respond again, then you know it's not going to work out. On one hand, if she does that, she may not be interested, but on the other, if she is, but is a total flake, that's going to cause you heart ache later..

The ball is on her side of the court right now. Let her get back to you. You can always try to follow up with her later in the week.

For now, just go do your own thing.

She said she'd let you , so give her the chance. She may have other things organized already and is trying to change her plans for all you know. Like Iwish says , the ball is in her court , you've let her know your interested.

Don't go chasing too eagerly , it can be a real turn off.