I have had this same problem with my 13 1/2 year old Lab(Dave). Dave has been a great dog, but has had severe arthritis for the last few years. In the last 9 months or so he has had Fecal incontinence. He would just be lying sleeping and all of a sudden wake up and not be able to get up quickly enough to get to the door, or would poop while he was lying there. What we have discovered is that he can not squat well enough while he is out. So I have taken to what we term "tail walking"Some people have thought this controversial, but we find that he walks well, and in a couple of minutes he will poop. The problem comes when they squat he just does not have the strength to hold up his back legs, will fall down and finish the poop. This has made life easier for him and us. We find that he only needs to poop 2-3 times a day, and a few minutes save a lot. I warn do not pull up on the tail too hard, but just enough to support. If you are fully supporting by holding up the tail than this can hurt your dog. Of course i would see your vet first to make sure that there is not anything neurologically going on.
We may not be able to reverse the arthritis but we can make her more comfortable and hopefully more in control of her urinary and fecal problems.
If she is not on any NSAID medication such as Rimadyl, Deramaxx, or Previcox yet than she should be. If she is on one of these medications ask your vet to either replace the NSAID with Tramadol or add it to her NSAID protocol.
In addition I have found that the following Chinese Herbal Formulas can help: Shen Ling Bai Zhu San for fecal incontinence and You Gui Wan for urinary incontinence. These are available on line and can be used together. Begin with 1/2 the dose listed on the bottles and gradually increase over a week or so to the full amount. A human dose and frequency would be appropriate for a large breed dog. Since these are herbs they can be used with any other medications that she is currently taking. Never use human NSAIDs such as Advil or Tylenol on dogs, because these can cause dangerous internal bleeding.
Herbs don't work as fast as drugs so please be patient. It may take 3 weeks or more to see an improvement.