The nausea can be a side effect of Imuran, yes. For many on here, their bodies eventually adjust.
However, let your doctor know about your side effects...especially if they become bothersome for you. He can either give you something to help with the nausea or readjust your dosage/how you take your medication to try and help reduce the nausea but still giving you a therapeutic amount of Imuran.
Sounds like you are also weaning off of Prednisone which can cause withdrawal symptoms in patients as well. If for some reason he thinks it's the prednisone he will keep a close watch on you and also may adjust your current dosage to help you taper down more effectively/safely.
Either way I would let your doctor know of your symptoms. Hope you start to feel better soon :hug: