Hi. My first thought as I was reading this was to chalk it up to the Prednisone taper. Joint pain is very common during and after the taper and can last several weeks to months.
However, as I kept reading, a few things struck me. First, I will say that autoimmune disorders can cluster, meaning that it is possible to develop more than one. ITP can be a symptom of Lupus, and they can each occur at any time, weeks, months or years apart. ITP is actually one of the criteria of a Lupus diagnosis. For that reason, I wouldn't quite call it an autoimmune storm. The two can go together quite often. I was diagnosed with ITP in 1998 and Lupus in 2006. That's fairly far apart, but many have the two diagnoses closer together. Prednisone can also mask the symptoms of Lupus and when going off of it, you can begin to feel the joint and muscle pain.
When I began to suspect that I had Lupus, my symptoms were like yours. I'd already had pretty bad muscle stiffness, but when the joint pain began, it started in my hips which lasted two weeks, moved to shoulders for a few weeks, went to elbows, etc. It moved around which is not likely with a Prednisone taper. Lupus can sometimes take a long time to develop enough to diagnosed. I had symptoms for a good year and a half before the specific labs began to be elevated. For some people, it can be much longer.
There is a difference between Prednisone taper pain and Lupus joint pain. When I had it in my elbows, I could barely straighten my arms out to push open a door. When it was in my hips, I couldn't sleep on my side. It does wake you up at night. Prednisone joint pain is bad when you first stand up and start walking and things like that, but it's not nearly as severe as Lupus joint pain.
You will need to see a Rheumatologist and have more tests to find out what is going on. There are more specific tests that will be helpful. Did you just have another ANA done? If so, do you know what the titer was? Do you know what other tests were done? It is helpful to always get copies of labs. I have a huge folder full of them and refer to them often.
In the meantime, try not to worry. I know that's easy to say, but you've had a lot going on lately so there is the possibility that this is due to the taper and the stress on your body from the surgery.
These are the criteria that must be met for a diagnosis. Usually if you have four of the eleven, it's good enough for a diagnosis. You do have to have either of these two antibodies: dsDNA or Anti-Sm, among other things.
lupus.webmd.com/tc/lupus-criteria-for-diagnosis-topic-overview
Please let me know if you have any other questions.