Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about Panda's screaming for no apparent reason. If he is eating and drinking normally then abdominal or gastrointestinal pain seem unlikely, but he sounds very uncomfortable. Dogs don't scream for no reason.
Unfortunately both Maltese and ***** *****s as breeds are prone to a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis. The symptoms may come on suddenly as they may occur when he happened to move just the wrong way, likewise one position may be comfortable but repositioning can lead to excruciating pain, thus the scream. This may just be a pulled back muscle too, but given the high incidence of intervertebral disc disease in these breeds I would rather be cautious and treat him as if this is more serious.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease. Of course an MRI is quite expensive so we often diagnose them based upon our physical examination and radiographs that are clear for other causes of pain.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for a few weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
If he is not significantly better with a few days of very strict rest then he should be re-evaluated by his veterinarian. The choice whether to have him examined today or not is yours. As long as he can move, and doesn't seem to be paralyzed he can wait until morning and see his regular veterinarian, but of course the sooner he gets started on pain medication, the faster he will be comfortable.
If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on his spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
He should be closely confined starting now. No stairs, running or jumping. If you have a crate for him I highly recommend using it. The less he moves around the more comfortable he will be and the faster he will heal. He should go out on a leash to relieve himself. Do not use a collar for him, a harness which more evenly distributes forces if he pulls on his leash is better. You will need to confine him for at least a week or two, even as he starts to feel better or he may reinjure himself.
Keeping him on the thin side is recommended to decrease stress on his back, but is no guarantee that he won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back or neck pain in dogs.
Please let me know if you have any further questions.