My dog would quite often sit down if you rubbed his back; now in the past 5 days he has started sitting down randomly in discomfort even without touching him. It seems to get better as he warms up (after 5 or 10 minutes walking), and he is running around still. He is also yelping when getting off the couch etc. But it isn't all of the time and more often when he hasn't moved in a bit. He is still eating fine but is being really clingy and acting out of character. He is a 2-year old rottweiler mastif. Anybody have any ideas what could be causing this?

Depending on what his growth patterns were like as a younger pup, my bet is on arthritis or a torn/hurt muscle or even ligament. Sore or injured muscles in giant breeds are very common. Mastiffs are also prone to grown disk displacement or general issues.

You should speak with your vet about glucosamine supplements for your dog, and might want o have an x-ray done to make sure it isn't anything too serious. Try to keep him relaxed for a few days and see if you notice any difference. Dogs with growth plate issues, usually live long, normal lives with small amounts of discomfort and usually only when they first get moving. I would also keep your dog off furniture, jumping off can cause un-needed strain to muscles. I like buying the big dog pillows from Cost-Co for $40.

It sounds like his problem is pain when he moves, which could be hips or vertebra, somewhere. Large dogs are susceptible to hip dysplasia. Dysplasia predisposes to fairly severe arthritis in one or both hip joints. Arthritis in joints in the vertebral column are also possible and painful, but opposed to other pain, arthritic pain is usually aggravated by rest or disuse of a joint, and alleviated by motion and activity... warming out of the pain.

Large and giant breeds are more prone to joint and ligament injuries when younger pups... joints, ligaments, muscles and tendons are not fully developed and the activity pups usually have can strain those structures to the point of injury that can create problems at maturity, if not at the time of injury.

To be more complete, one must also include intervertebral disk problems, but is more of an adult concern than a younger dog; and diseases of the spinal cord such as tumors are also a more remote possibility, as I said... to be more complete.

As to his being 'clingy and out of character', it most likely is because of his discomfort. He can't figure out what is causing it, nor where it is coming from. Hiding is one reaction from more intense pain, but he is seeking the comfort of being close to those he feels safe around... his family.

Most of these problems can be helped, comfort wise, with mild pain medication, and abstaining from doing things that hurt, like getting off elevated 'perches'. Your best advice, however, will be seeking help from your small animal vet, who can more accurately decide the problem and prescribe proper treatment.