A side effect like D, I think, would have shown up sooner than 3 weeks, unless you just changed doses. It could have been something you ate, but if its been going on for a few days, or as you say, it feels like C-diff, then it most likely is. You know the saying, Quacks like a Duck, most likely it's a Duck!
Your doctor can probably call the test in without even having to see you, so it really shouldn't be that much of a bother. Never hesitate to call with a big change in symptoms, as posted, its always better to catch things when they first begin.
CD 20 years officially, 30 unofficially. 3 resections '93, '95 '97
Managing with strict low residue diet, keeping symptoms to a minimum. All test show small amount of ulceration, still have occasional blockages. But still have a great time with my 2 daughters and husband!
Prednisone, 6MP,Prevacid, B12 shots, Bentyl, Xifaxan.....