There could mostly be 2 causes here:hormonal
and pathological.You need to get an ultrasound
and some blood tests for hormone levels including thyroid,prolactin
,blood sugar etc.The ultrasound will look for any pathology that might be present in the pelvis.The endometrial thickness should be noted.Diagnosis will depend on the findings.Fibroids,cyst,endometrial hyperplasia
are possible causes.