Like many here I do not relish the thought of going back to the doctor for yet another round of bloodtests and other kinds of testing, just because I have lupus and yet another kind of pain is rearing its ugly head. I hate it. At the same time I am grateful that I have medicaid to pay for all these tests, so they can be done, and hopefully one of them will be able to find some relief for the pain and other symptoms.
I have this pain sometimes under my right rib cage, and they have never found what was causing it, so they lump it in with other symptoms caused by Lupus. Because my first MRI revealed swollen lymph nodes, the doctors totally ignored the pain I told them about, and sent me to an oncologist, who kept sending me for more MRI's. Every six months I had this total body scan, for two years. then I had a heart attack, this ended the tests for the pain. I am still playing catch up, with the cancer doctor.
My pain however seems to be nothing to what you are experiencing now. It doesn't even seem related to what is happening to you.
My therapist however did have pain like you describe, and it turned out to be pancreatitis, and her diet had to change drastically. She didn't smoke, she didn't eat much red meat, but she loved her dairy foods, and she had to give them up. When she got married she was allowed to sip on a very small glass of wine to celebrate her wedding.
That this pain is causing you weakness also, sends warning bells off to me.
I think I could handle the "taste like" apple juice drink. I do not like ensure but have had to take it at different times in my life. Mostly to gain weight, and complete a healthy diet. I used to think just a few months ago that I sure wish I had those skinny days back;) 
Now Lupus is causing more shedding of wieght, and I am back to being grateful for being able to gain weight, because Lupus is taking away the pounds. And I will probably be wishing I could lose again too at some point.
When my therapist had pain similar to yours, she could not eat anything without experiencing the awful pain, so maybe your pain will not be pancreitis.
Hopefully yours will not be pancreatitis. maybe it will just be another strange symptom of Lupus, and you can at least be able to eat regular food. I don't know that I could have held out as long as you have without going to the doctor.
I think I practice a running scared phenomena. Afraid my pain is just another Lupus pain that cannot be cured, and at the same time i am running scared that it will be something else, and I will need more and more tests, just to make sure.
Be Well as much as you can. Don't worry about this or that thing that you might be neglecting because of this pain. Worry about you, and the best thing you can do to find some healing for yourself. I know this is hard, but sometimes we have to do what is best for us so that we can be there when our families need us to be there. One of my eight grandsons was here yesterday, I fed him big plates of beef stew, which he ate heartily. 
I don't think it matters to him how clean or dirty my house is. He does care, quite a bit however, for my beef stew;) I want to be here as much as I can to fix him this beef stew he loves so much.
And thank whatever powers that be, that today it is both mentally and emotionally a good day.
Take care. My prayers are with you.
Hester