A 27 year old woman who had delivered a female child 9 months back, came to the gynaecologist with complaints of absent periods since child birth. She has been using contraceptive methods as a family control measure. She was advised to get her serum beta hCG levels tested, which came out as 4.9 mIU/ml. Her prolactin level was 88 ng/ml, and TSH was 3.8. What could be the reason for amenorrhea in this case?