My mom has trouble breathing, but doctor says it's not asthma?

Many different cause: A number of things can be causing this problem. And some of them can be related to her age. If she is or was a smoker, COPD can be a consideration. Sometimes there can be an obstruction that mimics asthma but is not. A spirometry test can rule this out. Gerd has been associated with "asthma" as well. As you can see, many things must be considered.Talk to her physician.