I am so sorry you are going through this! That is such an awful and scary feeling to have to run to the bathroom, especially combined with pain.
In my experience, the 6mp didn't work at all for me - the prednisone I was on was what was calming my insides and once i went off that and was only taking the 6mp, I had another flare.
I was on Remicaide, which I developed antibodies to recently, and started Humira a couple months back (also took prednisone during that flare).
In my case, the prednisone made me lose weight, or at least kept it down - I lost a ton of weight from my flare and once I started feeling better I was eating a lot more, and I think the prednisone was keeping my metabolism going (or at least it felt like that with my heart constantly racing).
I hope the 6mp works for you, but one thing I would recommend is that when you go to the GI, don't take Immodium - let him/her see you at your worst so he/she can properly diagnose you. Also, tell him your concerns about pred and get an RX for anti-anxiety and sleep meds to help. Prednisone is, to me, the worst drug ever, but it seems to be th only thing that calms my intestines quickly during a flare.
Good luck!!! Try not to stress about this! :)