Ive been feeling very tired and drained can it be from cytoxan (cyclophosphamide)?

Maybe: The severity of side effects of cyclophosphamide (Cytoxan) depend on the dosage, and include anemia, low WBC, nausea, poor appetite, hair loss, etc. Certainly it will cause fatigue with exertion if you are anemic. In low doses, which I assume you are taking for SLE, it is better tolerated, but still you should be monitored. There are many other causes of fatigue. Talk to your prescribing doctor.