I fully agree with Dr Tan. Your mother will definitely benefit from a through review by her family physician.
Though I suspect the T wave inversion to be probably secondary to longstanding hypertension and possible thickening of muscle (LVH with strain pattern).
Also of note, T wave inversion can be seen frequently in the inferior leads (i.e. lead III and aVF) and does not always indicate pathology.
A repeat ECG with deep inspiration and expiration may be useful to demonstrate if the T wave inversion is dynamic.
Yes, ischemia can cause these changes sometimes, but is more sudden in onset generally and depends on presentation and other risk factors.
An echocardiogram will indicate changes of hypertension with thickened muscle causing both ECG changes and also swelling in legs (with age and hypertension the heart muscles are a bit stiff and relaxation can be an issue though pumping is well preserved).
As for weariness there maybe other issues such as anaemia, vitamin deficiency etc.
It would be best to have a comprehensive review with your family physician with further investigations as felt to be appropriate.