On methotrexate weekly 4 autoimmune. why might wbc drop to 3000 (low)one week & recover back up to normal 5000 two weeks later while continuing med?

Disease or medicine: Methotrexate can cause blood count suppression, particularly if given to patients with kidney problems. Some of the conditions for which Methotrexate is used, such as lupus, can also cause a low WBC count. Some healthy people, especially with east african ancestry, may have a lower WBC count than others. Minor variation from one test to the next is less important than the overall trend.

See below: If you had a viral infection, that could explain a transient drop in the white blood cell counts.