Yes, 6MP can make you really tired. I find it manageable on 50 mg, because I took 75 mg before and was almost comatose. Couldn't get out of bed, even 14 hours of sleep didn't feel enough.
The achy joint could be caused by tapering off/quitting Pred. I never had joint pain in my entire life, not even on Pred, but when I was done with Pred, I noticed slight joint pain that lasted a couple of weeks or so. It didn't bother me really, I only noticed it. Hope your pain is not too bad.