Well, there's one way to find out.....take only showers and NO baths. Bathing can cause UTI's anyway, that's why they suggest not doing this to people who are prone to them. Also, washing with soap in that area can also contribute, believe it or not. It has been shown that women who wash that area with soap and water are more likely to get an infection than women who simply use water. If you shower, the soapy water running down your body will be enough to clean you. You may, without even realizing it, be trying to keep yourself extra clean right now thinking that may help with the infections, when in reality you may simply be making it worse.
If you are one to take baths, you may very well have figured it out yourself - it could be your water supply as you say. But even if you don't take baths, it could still point to your water, Do you by any chance have well water?
Try no baths and see. We'll be curious to know if it helps.....take care.