I definitely agree, take her in to your vet to have her checked out. They will perform a full physical exam and will likely recommend some blood work to check for kidney, liver or thyroid disease, diabetes, urinary tract infection, etc. May be something easy to fix.
I have had many animals including Cairns and the vet is always a good answer. Since they are so expensive I will make an educated guess. Although Diabetic is possible my guess is Dysphagia. In most breeds it is any number of hereditary conditions that are breed specific. It is typically a muscle/nerve weakness that makes swallowing difficult. These disorders can be mild or sever and range from swelling in the throat or tongue to weakening of the esophageal muscles and nerves.
Are there any additional weakness in any other area?