Shortness of breath at rest is never normal unless it is an established condition of known cause, so it should be checked out as a matter of urgency by a doctor.
There are certainly many, many causes ranging from a simple infection to, as you say, problems with the heart.
The latter is the reason it should be checked out urgently.
Sudden shortness of breath can be a sign of a "silent" heart-attack; this is a heart attack that does not produce the classic symptoms of chest pain. Up to one fifth of all heart attacks are "silent" and they are more common in women. They also tend to be more dangerous, not in what they do, but because people delays seeking medical attention.
The usual procedure would be for the doctor to listen to the chest but usually unexplained shortness of breath at rest will result in a chest x-ray. As your mother previously had TB her lung capacity may have been restricted already, and capacity diminishes with age (this is normal, it falls after peeking at around age 20 or so), it may now have reached the point where it is leaving her a little breathless.
It is also possible to develop asthma or an allergy at any age.