Your mother should contact her doctor to rule out any serious problems such as heart failure. Also, there could be side effects from the medication so if there were added medication or an increase of medication that could be the problem. Also, there could be a respiratory problem that should be ruled out.
Years ago I had a dry cough due to heart failure, but it usually happened at night while in an inclined position. There were other symptoms that included faster than normal heart rate and shortness of breath.
Thanks for sharing and if there are any followup questions you are welcome to ask. Take care, and I wish the best for your mother