Is she no longer interested?

Since communication is so important in a relationship, and this communication seems to be fading, if I were you, I would be upfront with her about her feelings. Not deep and committed talk about marriage, just ask her something like you are feeling. Don't text her. Don't IM her. Don't email her. Talk to her on the phone or in person. Say hi ___, I like talking with you but it feels like you've been a bit distant in the last week or so on our calls. Are you feeling like you want to take a step back from this for a bit? If you are interested, I'd still like to take you out. Keep it short and sweet. Don't go into detail. Don't explain why you think she's distant and all of that. Say a couple of sentences and then LET HER TALK. If she tells you that she's sorry it appeared that way, and she just has a lot on her mind, or is knee deep in some personal issue, then give her space. Tell her, OH, well I'm here if you need anything, or want to talk, but it seems like you might want some time alone. If she says, well, I don't think I want to go out with you, or i'm not sure if we should go out, or something like that, save your dignity and your self esteem without being rude and just say "oh, ok. I appreciate you being up front and sharing that." Then YOU take the lead in getting off the phone so you feel better. Say "well, OK, cool. Let's move on from this, but I wanted you to know it was great seeing and talking to you again, and I wish you all the best!" BYE! Then, wipe your hands from this and go get the girl that is out there waiting for you. : )

Hmm. I know I usually don't have a lot to say when I'm upset/troubled by something - even to the guy I like. Maybe something is bothering her? Usually people become withdrawn to everyone around them when something is up.

However, if you want to show you care, I think you should call her and ask her what's going on. Not in a clingy kind of way, but just check up on what's been going on in her life. If anything is up, ask her if she wants to talk about it.
(:

Ok. One rule
don't be clingy
girls get scared off if you text, phone or talk to them too much
give them time to miss you. And they'll appreciate you and your texts all the more
it gets boring if we know when exactly a guy is going to text us :)

Just give her a call and talk to her, tell her how you feel, don't hide anything. She needs to know and you need to get your questions answered but the only way you will know is if you ask. The worst she can say is I'm not interested. She probably just has things on her mind but I can't really say that since I don't know her personally.