My mastiff is four years old she never makes messes in the house anymore, but lately I have found she's been urinating while laying down in the house. The back door is always open so she can go out whenever. Once she was laying at our feet while we were sitting on the couch and when she got up there was a puddle on the floor. What could be causing this and what can I do to help her. Thank you!

I had a mastiff and this happened to her as she got older. It was a weak bladder/loss of bladder control issue. It sounds like you mastiff is to young for this to be age related, and you might want to go to the vet and check for any treatable condition she might have. Good luck.

If she is urinating while laying down odds are she either can't help the urinating or she doesn't know she is doing it.

Best to check with your vet to make sure she doesn't have a urinary infection or other underlying problems.

Most large breed female dogs get this. It may be a sign of infection. Or she may need to be "tightened up" My dog had this and was given a pills that help the muscles tighten up and the leaking stopped... in fact that has happened to all of my female dogs.

Kind of have to disagree with the most large breeds dogs get it... sorry :) my mum has had Great Danes for 20 years and never had a dog do this.

However, I have a miniature poodle who leaks urine in her sleep.

The pills that they can take have a lot of bad side effects which is why I did not give them to my dog.
Some dogs may not experince as many side effects though so it is an option.

Just stated what I knew from my experience as well as others I know. Sorry! I did not know there were side effects... still I would go to the vet and get this problem evaluated

It is not uncommon in older, spayed female dogs. Urinary incontinence, that is.
There are medications to treat it. The most common is Stilbesterol (diethylstilbesterol). It's a nonsteroidal sex hormone, relatively similar to estrogen.
Chances are if your dog is put on this drug, they will want to to medication monitoring bloodwork usually every 6 months as it can very rarely cause poor production of blood cells, so they want to check for things like anemia.

I believe there are other medications, this is the only one I am familiar with.

Just looked it up - the other drug is phenylpropanolamine (PPA), but it's being phased out I believe.