Very heavy reading and page turns and charge cycles can do it. My first kindle was replaced by amazon 2 times because I was not getting the battery life claimed by them.
It was not a indexing problem or leaving the radio on. It was me, I was reading with the kindle on for very long stretches. I am a fast reader so I blow thru pages at a good clip.
If nothing has changed in the way you use your kindle and your reading time is the same. But the battery is not lasting as long between charges, I suspect your battery is getting weak and starting to go down hill.
I have replaced my k3-3g's battery once in the two years I have owned mine. They age and can go bad even if new and never used. So use will vary from one battery to another.
Good news is that is very easy to replace your battery yourself once it gives its last gasp, tho I recommend you have one on hand before it goes belly up on you. So you do not have to go through reading withdraw and start reading labels on cereal boxes, to get a fix.