A few questions about Imuran

Hi everyone my rheumatologist prescribed 100mg Imuran daily for me. I started taking it on Friday and I've noticed a few problems such as stomach upset and paleness. Ive also been experiencing very very severe joint and muscle pain and wasn't sure if that was a side effect of the Imuran. I usually have moderate pain maybe once or twice a week but nothing like this! Has anyone else experienced this? I also take 400mg of plaquenil. Also, did imuran work for you and did the benefits outweigh the risks? I appreciate any answers..