At such a young age having poor memory is not common. If it is pertaining only to learning and not impairing your activities of daily living then not to be worried about. It may be because of poor attention to a given task which is reading in your case.
Poor memory is a speciality of neurologist.
Deficiency of micronutrients like
vitamin B can cause poor attention and memory issues.
Hypothyroidism is a cause for poor memory but in your case I do not think It will play any part.
Next Steps
check
vitamin b12 levels.
need a consultation with specialist nearby for detailed examination