OK,
So I've been seeing a girl for about 4 months I'd say, Things seemed all right for a while but have been deteriorating in the last month or so.

We spend a day together about every week or so, but in between we speak on the phone, and she comes into my work and we go out for a cigarette and a chat, other than that, we SMS each other... around 15 times a day or more is the average.

I saw her this Monday gone, we had a great day and she seemed to want to be close to me (touching my arm, resting her legs on me as we watched a DVD). She texted me that night. Since Tuesday morning I've sensed a distance from her... her tests on the Tuesday were few, and seemed rather... brash? Rough?. just in the wording I got a vibe.
Wednesday came, more "brash" texts and a quick phone call, everything in her voice sounded normal...
Thursday, she called me in the morning out of the blue for about 20 minutes, then I didn't hear from her until 9pm that night.
Friday a couple of messages, but nearly nothing... she came into my work and we had a cigarette together, things felt great! She hugged me twice (strange for her as in public she isn't very afffectionate) she smiled the whole time and softly complained with "nooo" when I told her I had to go back into work. Friday night no texts.
Saturday (today) one text in the morning saying she had a headache, and nothing since... it's now 9pm...
One more thing, she dropped her phone into a puddle a few weeks ago and it has been playing up ever since, but even after that she would text me around 10 times a day... it's now been diminished to very few in a matter of days...

Things that are going through my head:
Why would she be ignoring me?
If she didn't want to come and see me yesterday, she wouldn't have.
If she IS ignoring me, what should I do?.

Part of me wants to ask her why she is ignorning me... Part of me wants to ignore her back, and another part wants me to say something like "text me when you can" then try to put it out of my head... thoughts?

Could it be that she's just relaxing into the relationship?

The only way to find out for sure is ask her.

Just be straight up and honest.

I think she may feel somewhat like I do about texting.

Once or twice a day at the most.
What do you say when you text someone 10 or more times a day?
"I'd miss you if we were not in constant contact."

Come right out and ask. Get to REAL communication. Talking face to face or on the phone.
Body language and tone of voice add a great deal to communication.
Try it. The human voice works better than txt.

Text are not a date or show of interest and take no effort at all. You need a better plan, man. Something more than a cigarette and a chat at your lunch break at your job.

Why the freak can't you return the favor? Why does she have to do all the work? Why does she have to chase you down?

I wouldn't talk to you either. Come on guy, I know you know better! What you think your gods gift or something? Show some effort, do some work.