Welcome! Narrow stools are typical IBS symptoms. Make sure you are drinking tons of water. The number one reason people become constipated is because they are not drinking enough water. FiberCon may not be the best thing for you. With IBS C, it's important to be careful with the type of fiber you take. It may be that Miralax would work better for you right now, until you are having more regular BM's. When you are ready to introduce fiber into your diet, it's always best to get it through foods. If you require a fiber supplement, Benefiber is a soluble fiber which works quite well for folks with C. Good luck!
Lower back pain is actually the #1 reason for doctor's visits. Little tidbit I learned in a few classes recently. It may have very little to do with the GI issues, but then again it may - there's a lot of nerves coalescing in that area and sometimes they course together and you can get referred pain.
Any fiber supplement will give you some initial gas but after a while it should ease up a bit!