What Causes Reduction In White Blood Count With History Of Lymphoma?

my aunt had lymphoma but it was treated. she has not had any chemotherapy for last few months. now her white blood cell count is 1.0 eventhough she had three injections for increasing them two weeks ago. when released from hospital it was 3.7 now it is back to 1.0. what could cause lowering her white blood count? bone-maro test was negative for any cancer...thank you