I haven't had the SBFT yet myself, but I've been through a rather similar situation to yours. All the tests I've had (including bloodwork, multiple colonoscopies, etc) were normal or inconclusive, but I have all of the "red flag" symptoms of IBD except for fevers (I have had chills, but not actual fevers oddly - I've also had unintentional weight loss, blood in the stool, getting up in the night to go, etc - so it's not IBS because of those symptoms). My doctors ran various other tests to rule out things like Addison's, RA, Lupus, thyroid, etc. When all that was ruled out, I also asked to try a pred trial - it worked fabulously, so then they tried Entocort on me, and that worked nicely as well and got me into remission. So, because most everything else has been ruled out, and because I respond so well to the IBD meds that I've tried, I basically have a pseudo-diagnosis of exclusion right now. My doctors take me seriously and treat me as though I have IBD.
I realize that's not particularly helpful, but I would encourage you to do the trial of pred if you can. The doctor who prescribed it put it bluntly - he said pred doesn't work on IBS but it does on IBD, because it only works on inflammation. Well, it worked wonderfully so we know that I must have inflammation hiding somewhere! So at any rate, trying pred could be very illuminating - at the very least, if it works, you can essentially rule out IBS and narrow your focus to inflammatory conditions. With the fevers and weight loss you mentioned, that doesn't sound like IBS and I would imagine that pred would have a good chance of working for you.
Oh, and don't feel bad if the Hyoscyamine doesn't do much for you. It's primarily an IBS med, as you said. It actually made me way worse - it caused me terrible nausea and vomiting. I stopped taking it right away but the wicked nausea lasted for a couple more weeks after I stopped it - it was horrible! And my GI said he hoped it would "calm" my bowels - hah, no!
I hope that was a little bit helpful. I'm so sorry you're in a similar situation to mine, but I can tell you that there's a light at the end of the tunnel, and for me that light started growing brighter when I did my pred trial and it was successful. remission is possible even without a firm diagnosis, I'm living proof of that. Hang in there and good luck if you do try pred!
