You have been started on iron supplements
thinking of iron deficiency
anaemia.It also needs to be searched for cause of iron deficiency whether nutritional deficiency
or secondary to blood loss.
Low hemoglobin could be due to other causes like B12 or folate deficiency
.This needs clinical correlation and confirmation by blood tests such as MCV,MCH,MCHC,serum iron,ferritin,transferrin,B12 levels and stool for occult blood.
However you have been started on iron supplements means you must having iron deficiency.
Three tablets of ferrous sulfate
325mg per day are more than enough for hemoglobin of 8.7.
Hemoglobin takes time to increase.It increases by approximately 1gm per week if there is no active loss blood simultaneously.
Hence continue with the same treatment for one month and repeat HB,CBC after one month.
Hope this could help you.