It would help to know what kind of arthritis she has and what tablets she is on. Painkillers for osteo arthritis are very different to say immunosuppressant medication for inflammatory arthritis.
The level could also make a difference as to whether it is just a bit lower than normal (but still in the normal range), worryingly low etc... As rojash says, certain conditions, like rhumatoid arthritis, can cause a low white blood cell count in themselves.
Some arthritis meds can cause low white blood cell count but so can many other things and if a patient is on certain medications with certain risks there is often monitoring done such as with DMARDS for rhumatoid arthritis (Sulfasalazine, Methotrexate etc... as bednobs says).
I have had to have two weekly blood monitoring for an arthritis drug to check for possible problems and monitor inflammation levels.