Hello again, and thanks for your reply with additional information.
From your description, it sounds like she may have a collapsed trachea, but that kind of cough usually sounds like a goose honk and the loss of elasticity in the rings of her trachea, might have shown up on the x-ray, however, this is not easily seen in all dogs during x-ray; an endoscopy would show more. Pomeranians are predisposed to this condition. A slight trauma to the trachea, which was incurred accidentally and you didn't know about, might also have started this coughing.http://www.sniksnak.com/doghealth/col-trachea.html
She also could be 'reverse sneezing', as this can sound like a cough and have the properties you describe. This is not a sneeze at all, but a spasm caused by an irritation of the soft palate.
Coughing can also be related to a cardiac condition, so because she's a 'senior' and she seems to need to put herself in a more 'upright' position, to cough and try to clear what's bothering her, perhaps an evaluation by a veterinary respiratory/pulmonary and/or cardiac specialist, would be a good idea, at this time. Specialists can be found at ER vet centers and also Veterinary Colleges. Ask your vet for a referral, if you decide to do this.
I hope you're able to have her definitely diagnosed and treated soon, so she'll be feeling more comfortable and you'll be feeling less worried!