Guitar neck changes while playing...?

It feels like after I've played my guitar for half an hour or so the
neck actually bows a little...as if it's giving in to the extra
tension from the strings (from bending perhaps?) I thought I was crazy
so for the last few days I've been testing it. When I first pick it up
I grab the G, 15th fret high E string, and bend it up to an A. Once I
hit the A, I let my finger slide on to the B string which is also bent
as it moved when I bend the E. This transfer is very easy...in fact,
it almost happens on its own. After a while I do the same thing, but
usually have a lot of trouble making the transfer from E string to B
string, but only around the 12-15 fret area, although every string on
almost every fret feels a little higher.

The guitar is about 9 years old, and I'm afraid that maybe the quality
just isn't very good and the neck is starting to "go"

Anyone else experience this?