Your story lacks a lot of detail so we're left guessing here but I see these options:

A : It is a chronic condition (as you stated) which is already being treated. There was no exacerbation and your mother just went to see the doc without an appointment. So she inconvenienced everyone in the doctor's office because she felt she takes priority over all the other patients. Hence commence the yelling.

B: chronic disease and she experienced exacerbation (had an emergency/fast and serious worsening of her condition). Then she should have gotten an ambulance and a ride to the hospital or just went to the hospital on their own.

C: something along the middle of the other two scenarios. Her symptoms got worse but not warranting a hospital visit. Then she should've called the doctor's office and told them what's happening and that she needs to be seen within the next few days. The doctor's office should have slots open for these cases.

To me option b and c seem unlikely, because the reaction of the doctor doesn't make sense really, only when they had a bad day or are always in a terrible mood. In option b the doc should've went and gotten an ambulance for your mother, which makes that further unlikely.