I don't think it has anything to do with E-Coli. Many people have experienced diarrhea
when doing UT men and women. I think it has more to do with extra salt being injested at times. I have read on a UT site that when doing UT and you get extra sodium in your diet then it will cause diarreah. It sounds like from what you said you are a very clean person and for it to be caused by E-Coli would seem unlikely especially when not experiencing any UTI or bladder infections.