If the prednisone is working, your joint pain and numbness/tingling should decrease. If you don't find anything has changed in 7-10 days, check back with the rheumy. 15 mg isn't a very high dose.
It is very common to be partially diagnosed with Lupus/MCTD -- actually more common than not. The diagnosis itself is not the most important thing at this point -- having a rheumy who treats you is most important!
You are on the right track!
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2002DIAGNOSING LUPUS & LUPUS RESOURCES
"Life is far too important to be taken seriously." - Oscar Wilde