Is it ok to exercise during a lupus flare? I felt great until I caught a viral infection a month ago, now I have worsening joint pain/swelling/stiffness, fatigue, malar rash and swollen lymph nodes.

Generally, yes. As long as it does not make the symptoms worse, light exercise may help improve the symptoms from the lupus attack - especially a mild one. If you have doubts, contact the physician that treats you for the lupus.
Rest. During such a flareup when so much inflammation is present, i would not exert yourself but, instead, get plenty of rest and take the medications ordered by your rheumatologist. Avoidance of emotional stress is also important.