the curve in your eyes can change which worsens your vision. your vision normally stabilizes by age 30 or higher which im sure your mom has reached after that it will unlikely change again until you reach old age.
if you noticed before you got contacts every year you got a worse perscription from your doctor anyway for glasses.
if you have been wearing the same pair of contacts for 4 years its time to switch. otherwise your doctor (if you have been getting a yearly checkup) isnt giving you the right prescription.
Or they are not giving you lenses made for astigmatism