Since this trigger for your mother was the shorts splitting and they were 5 years old, would it not be a proactive step to look at the clothing you wear often to make sure it is sturdy enough for the stress of bending/stretching? A lot of clothing can't last 5 years, and perhaps if they had been replaced a couple of years ago they wouldn't have split. Also, are they the correct size? Are you the same size you were 5 years ago? That has some bearing on the incident that caused this latest flare-up.

I don't know what 20-ish stone is, but a good gauge of where you are health-wise would be a BMI calculator that takes your height and weight and converts it into a Body Mass Index. This is one from NHS. You can also discuss how healthy you are with a physician and this may assist in getting her to stop commenting if you are truly in good health.

Are you aged 18 or over? - Calculate your body mass index (BMI) for adults - NHS