You do not have pancytopenia but thrombocytopenia (decreased platelets) as your haemoglobin and white cell count are within the biological reference intervals. Your symptoms of mucosal bleed are due to the low platelet count.
Pancytopenia (bone marrow failure syndrome) may be due to inherited or acquired disorders that affect the hematopoietic stem cell. This may involve either one cell line or all of the cell lines -red cells (anaemia), white blood cells (leukopenia) or platelets (thrombocytopenia). The most common cause of acquired bone marrow failure is aplastic anaemia but it may also be secondary to myelodysplastic syndrome, paroxysmal nocturnal hemoglobinuria or some malignancies. The damage to the progenitor cell may be qualitative or quantitative and the cause could be a congenital defect resulting in inherited forms of the disease or due to exposure to a noxious substance like viruses, toxins, or chemicals. There may also be immune-mediated suppression of hematopoiesis (blood cell formation).
The treatment is based on the underlying mechanism thought to be responsible for it. Hematopoietic growth factors and androgens have been tried in an effort to stimulate hematopoiesis and immunosuppressives are used if an immune mechanism is suspected. You should get a bone marrow examination done to assess cellularity & morphology of residual cells along with cytogenetic/molecular studies to document any chromosomal abnormality.