I definitely know where you are coming from. For me, hi fiber foods make me sick. I have pretty much sworn off vegetables, which is very hard for me. However, metamucil seems to work well for me and I try to take it at least a few times a week.
Yes, when having a flare or when i am constipated i definitelt feel it in my lower back.
What fiber supplements have you tried? If you are able to eat whole grains or vegetables, I would start there and try to add more without using a supplement. If that isn't an option, id use a powder to be able to experiment more with the amount of fiber. And finally, check to see what kind of fiber is in the supplement and try different ones. Good. Luck,!

Have you been checked recently by a doctor? If fiber is your issue learn to add more gradually to your diet. Eat the grains that you can - I don't have issues with fiber causing problems so I can eat nuts and be fit as a fiddle. Metamucil was recommended when first saw my GI it never caused me any problems very gentle... start out with 1 or 2 capsules till you see how they make you feel. Adding fiber foods is generally easiest to do - changing your diet up helps too... Are you eating too many processed foods, soft foods, white breads cut back on these and add whole foods veggies, fruits, whole grains, cereals... fresh juices (w/ pulp) adding these changes should keep you running smoothly with out the back pain. Gradually adding them is key... Hope something helps!

I take Questran powder to help bind my foods. Ask your doctor about that drug. I have had three resections and I have to take two large scoops of powder mixed in a small glass of water before each meal. It has no real taste to it. They also have an orange flavor. See what your doctors thinks about it.

Is it all fiber that makes you ill? Soluble fiber, like the kind found in Metamucil, is a soft fiber that is easily dissolved in water. Insoluble fiber is much harder and difficult to digest. Maybe soluble could be helpful?

I've had irregularity for years and many doctors have recommended increasing fiber and taking Mirilax, this was prior to being diagnosed with crohns. Taking Mirilax required that I stay next to the bathroom all day. I've always had a very high fiber diet but nothing made me regular, 3-4 days between bm. :{ Docs say this is normal but I guarantee that if they were that irregular they would think otherwise!

Anyway, I've started taking colace, one jel cap in the am one at night. It has helped more than anything else I've tried and initiates daily bm's, which alleviates lower back pain. You can pick colace up at any drug store for a few dollars.