With my dog Cadence, she only limped when she was just casually walking around the house.  I was taking her on long walks and she never once limped on the walk.  When we would get home, and she would walk around, that is when I noticed her limping.  I would touch her leg and it didnt seem to bother her.  The limp got worse over a few days and when she would sit, the bad leg would go out to the side.  Thats when I decided to take her to the vet.  Make sure you check the leg to see if you notice any protruding bumps.  I thought it was just a sprain but the vet immediately noticed a bump (tumor) when she looked. When she showed me, I then was able to see it.  I still kick myself for not noticing it sooner.  Just keep and eye on your baby and make sure she doesnt over due it.  

For Jackson, there was an immediate connection to a rough play session of chase in the yard.  He bit it on some loose soil on a tight turn.  He whined right then and held up his back right leg.  He ended the game and wanted to come inside and rest.  So we had that connection that told us it was a soft tissue injury.  We figured he pulled something up in his hip.  We rested him (5d) and gave rimadyl and the limp went away.  But we didn't ease back into play, so immediately the limp came back.  You'd only see it when trotting outside on a walk.  On a normal, slow walk = no limp.  No limp when playing inside or tug 'o war.  We always thought it came from his hip, but it's only looking back that we now know it was his knee.  Well, come to find out, cancer had been growing at his knee, weakening the joint, stressing the tendons, which eventually led to a full ACL tear.  If we hadn't decided to go with the ACL surgery, we wouldn't have found the tumor for months, until it got bigger.  Like your pup, the limp never seemed to bother him.  It went away the first week of rest, but came back and stayed no matter how much additional rest or rimadyl given.  I guess from our story, I'd encourage you to feel her knee, just above and below it.  Feel both knees to get an idea if there are differences.

Good luck with your doggie!  Keep us posted!  Let's hope she's the actress you think she is.

Great!  Keep a good eye on her and hopefully we won't hear from you again!  That is said, of course, with the most love in my heart!!!!  She's wicked adorable!!  Belly rubs to all.

I know this is an older thread, but I thought I'd throw in our experience for other dog owners who might be searching here for answers.

Norah's situation started with an on-again-off-again limp.  There never seemed to be a really distinct correlation between activity and limping, though we did find that our walks were typically limited to about half of the normal distance before she seemed to be in some pain.  Initially the vet prescribed pain medication (tramadol) and rest, but after no improvement for a couple of days we had xrays done.  The end result is that she had an osteosarcoma in her right hind leg.  There never was a visible lump or anything of the sort.  The only indicator that we had was her limp.

In short, I would give a dog a couple of days rest if she's limping and, if there's no improvement, go see a vet for some diagnostic tests.  I never thought that such a seemingly minor symptom could have such a massive effect on our lives.