How old is she?

Could be really bad. Take your pooch to the vet immediately.

she's a french mastiff 8 years old

Let her outside. With my dogs, it's almost always that they have to pee or poop.

Give her a good brushing and a cool bath. Give her a place to sit in front of a fan. Take her to the vet. Look for an outside stimulus (animal, activity) that may be stressing her. Massage and calm energy. Check food digestion, poop. Make sure she has plenty of water. Take her for walks every day to work out her joints.

Sit with her, interact with her (petting, treats), and try to read what she is feeling.

Just read your last post. I would start with therapeutic exercise and massage. Age may be making her uncomfortable.

Panting and pacing can be signs of pain. I suggest you take her to the vet to rule out any physical problems. She could also be showing signs of dementia, especially if she's a senior but, with my elderly dogs that had dementia, I never saw them panting when they paced. There are meds that can help if she has dementia. Also, be sure to ask the vet if your dog needs some Pepcid for gerd/reflux.

What is the weather like? Do you have the air on or is it hot and humid inside? There's not any information to work with!

My dog died of liver cancer. He was 12. There are no symptoms until they start to pace and pant because the liver tumor starts to hemorrhage. We knew that our pooch had liver cancer about 6 months before he died because we had blood work done on him and his liver values were elevated. We did further testing that confirmed the cancer. The vet explained to us that, most of the time, liver cancer is not detectable. It usually has no symptoms until the tumor starts to bleed. When that happened, my boy started to pace and pant all night long. Take your baby to the vet.

jilldag did your dog pace and pant all day or just at night? our dog just does this at night, during the day we walk, go to the park, she eats her food, she poops

My dog was much better during the day. The last day before he died, I was up all night with him. My boys had a soccer game and it was only 5 minutes up the road. My pooch had been suffering all night for about 5 nights. I had no intention of taking him to the game because he was not doing well. We knew the end was coming, but we couldn't bring ourselves to put him down. He was my sidekick. I lost him a year ago and I miss him desperately every single day. (I found one of his hairs in a purse that I hadn't used in a while just yesterday, and almost lost it.) Anyway, my plan was to run my boys to the game, and come back to stay with my pooch. Even though he had suffered all night, when he saw my boys with their soccer jerseys on, he got up and went to the door. He decided that he was coming to the game, and I brought him with us. Dogs are curious when it comes to pain. They seem to suffer significantly more at night than in the day. Maybe the daily distractions make their pain more bearable. I don't know. Hopefully this is not what your doggy is suffering from, but I think that the symptoms do warrant a vet visit.

snookums2 its 63- 65 degrees room temp cool and foggy here in San Francisco at night , heat not on , so she is not warm

Could be a cognition decline/impairment

http://www.aspca.org/pet-care/dog-care/common-dog-behavior-issues/behavior-problems-older-dogs

With nothing to go on other than "dog pacing and panting at night" there is absolutely no way to diagnose the specific cause on-line.

Some of the above suggestions are seemingly excessively worrying the OP - relax, take your dog to the Vet.

Pacing & panting do indeed signify pain.

could be heart problems, could be thyroid, could be anything, & none of us can diagnose via internet.

The behavior is disturbing in any dog, & in an older giant dog, it's downright alarming.

Please go to the vet before you read another line.

I wish you & your beloved dog the best.

Have you taken your dog to the vet?

It would be helpful to others to hear what the vet said.