Neva, C. diff testing requires it be done in triplicate. It can be missed in a single sample. You may have to take probiotics for several weeks to note a difference. In addition, some probiotics work better than others for some individuals. You may want to consider trying a different brand if one doesn't seem to help.
IBS is a 'generic' catch-all for 'this person has a GI problem, bowel issues, pain, and we don't have any test results that show anything.