Thanks for answering my questions. Panting in dogs can be caused by a variety of things, including pain, nervousness, heat, stress, as well as lung or heart problems. We are always more concerned when we are dealing with an older pet such as yours.
If you think she is having labored breathing rather than simple panting, that would be considered an emergency situation and I would not recommend waiting until your appointment tomorrow. If she is having difficulty breathing, she should be seen by a vet as soon as possible. She may need to be placed on oxygen while the underlying cause for the problem can be sorted out. Since I can't see her, I cannot tell you whether this is just heavy panting or truly labored breathing, but if you are unsure, err on the side of safety and have her seen as soon as possible.
If her panting is quite loud, this usually suggests a narrowing of the upper airways. A common cause for that in an older dog would be a condition called laryngeal paralysis. This means that the vocal folds in the larynx that normally move in either an open or closed position depending on whether a dog is breathing or swallowing are not moving properly. This makes the breathing very loud, especially if the dogs is very active. The breathing usually quiets down and may be nearly normal when she is sleeping though, so try to watch for that tonight - when she is sleeping - take note of what her breathing pattern is and be sure to pass that information along to your vet.
Anything that is causing a narrowing of the airways such as a mass lesion in the throat or neck or lymph node or something pressing into the airway could cause some panting. Because of the drooling you mentioned, be sure your vet gets a good look in the back of her throat - this can be challenging even in cooperative dogs, and may require sedation in order to make sure there is nothing back there causing a problem. Be sure to mention the drooling to your vet tomorrow - it may or may not be related.
Most heart problems (including heartworm disease) will cause more coughing than what you are describing, but this still needs to be considered.
Primary lung problems can also cause a lot of panting. This would include things like fibrosis (scarring) of the lungs, infections (fungal would be more likely than bacterial since there isn't too much coughing) and unfortunately cancer.
Another condition in dogs that can cause them to pant a lot is Cushing's disease. This is an increased functioning of the adrenal gland and the changes in the hormone levels that it secretes can cause the panting. Most dogs with Cushing's will have shown an increase in water consumption, so if you have noticed this, be sure to mention it to your vet as well. Another hormone imbalance called hypothyroidism (low functioning thyroid gland) can make dogs less tolerant of temperature extremes so that may be something to check out too.
Pain anywhere (including arthritis type pain) can cause dogs to pant a a lot. so a good general physical exam looking for sources of discomfort should be done.
I know it is hard not to worry when you have an older pet, but you are doing the right thing getting this checked out by your vet. After a physical exam they will probably want to do a chest x-ray and if there is no current heartworm test, they should check that as well. If the chest x-ray is normal and/or if you have noticed things such as an increase in water consumption, they will probably want to do some blood work to check for the hormonal imbalances I mentioned above.
If your vet seems unsure whether there is anything significant on the chest films (these can be difficult to interpret sometimes) ask them to send them into to a radiologist for a specialists opinion so you can make sure nothing is being missed. Most vets are happy to do this (although there will probably be an additional charge for it)
I hope this gives you the information you need.