low blood counts and leukemia

I am a health 28yr old female. I have had ongoing problems with a low white blood cell count (neutrophils and lymphocytes) for over a year and was referred to a hematologist. I have however becoming anaemic (low hemaglobin, low hematocrit, low mcv and mch) in a short period of time where my hemoglobin levels droped from 12.5 to 9.9 in a matter of weeks and then down to 8.5 and have struggled to get back to normal levels. I am not responding to oral iron(I have been taking this for 3 months) and an iv infusion of iron has helped my numbers creep up to 10. My ferritin levels are high/normal and my hematologist said this means it's not iron deficiancy. I also have waxing and waning lymphnodes in the cervical area. I have been suffering night sweats over the last couple of months, fatigue which feels like exhaustion and terrible feeling of fullness and nausea to the extent that I can only eat very small amounts and usually have to stick to soups/liquids. (GI investigations gave me an all clear). My white blood cell count is still borderline it sits between 3.5 and 2.5 and my lymphocytes and neutrophils are always low. My hematologist suggests a bone marrow biopsy, would you be able to explain what they would be looking for with my symptoms and test results.