Lynn, in my experience, when a good kid suddenly acts up like that they are usually sick, even if they don't know it yet. The fact that she's telling you she doesn't feel well means you should take her to your pediatrician as soon as possible.
Another possiblitly is that she's on a developmental cusp, which can also cause otherwise well-behaved kids to act up suddenly and seemingly for "no reason". But rule out medical problems first. It's also possible that she is having "growing pains" -- which are very difficult for a child to describe. For the moment, trust her that she "hurts" and see the doctor. Best luck!
p.s. Also, it IS possible to have an infection without a fever that registers. When I was a child I had strep throat for MONTHS with NO fever, and no sore throat, which can be extremely dangerous if left untreated.