Air Nomad—take a deep breath!
Slow down a little bit ... everything will be okay.
Since you DID have a bout with breast cancer, I'm assuming that you keep regular appointments with your doctor. Should anything spring up again, I'm sure they would be able to detect it.
The diarrhea could MOST DEFINITELY be from drinking a litre and a half of cranberry juice. Make no mistake.
If it is a UTI, one night and morning of drinking juice isn't going to make it feel better. If you're really that concerned, this is something that can be easily and cheaply tested for at a walk-in clinic. There is no need to go to the ER. I had what you're describing once and I thought it was a UTI as well. Turned out to be bacterial vaginosis, which surprised me because I'm not sexually active. The symptoms can be quite similar and the treatment is the same—a week of antibiotics (and more cranberry juice).
Also, we're around the same age, and I have to tell you, when 30 hits, weird things start happening in the reproductive area. I've started to have discharge and pain between cycles that I'd NEVER gotten before. Had a bunch of (very expensive) blood tests and even a transvaginal ultrasound—and nothing's there, just changes in the body. So, it could be a UTI, it could be BV, or it might be nothing.
Are you taking any steps right now to treat your anxiety? Even by the tone of your post I can tell you're super-stressed. Even though a full-body MRI (which you need a MIGHTY good reason to get, by the way) might sound more reassurance, I promise you that proper anxiety treatment will make you feel a lot better in the long run.