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Normal temp is 100.5-102.5 for a dog . Anything over 103 really warrants immediate medical care. The panting and lethargy could be due to the elevated body temp. If she is also having some incontinence this combined with the elevated temp could indicate either a urinary tract infection or kidney infection. Kidney disease can also cause weight loss.
Now in some cases excessive panting can also cause an elevated body temp. Panting can be a sign of pain, trouble oxygenating or it can be a symptom of a disease process such as hypothyroidism or Cushings disease.
At her age it would be impossible to say for sure what is going on. I know no one comes her to the site to have someone say that their pet needs to be seen by a vet but in this case there are several factors that warrant a hands on exam.Her age, the panting and the elevated temp.
You vet will likely want to run blood work, urinalysis and make sure there are no problems with the heart. Depending on what they find on physical exam they may also want to get some x rays.
I would try to get her in to the vet as soon as possible. If going to the ER tonight is an option that would be best. If not I would have her seen first thing in the morning.
I hope this helps give you some direction.
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