Could be a kidney stone. You should go to doctor asap . Be careful with banana only diet, bc they are full of potassium and too much potassium can be bad for your body.
It sounds like you could have an infection, maybe urinary tract or kidney. I would call your doctor. Also, that diets sounds really unhealthy! There's no way your body is getting everything it needs from bananas and water. I would stop that and if you're trying to lose weight, just watch your portions and eat fresh foods as much as possible and cut processed "foods" out.
You absolutely have an infection! See a doctor as soon as possible!
I don't believe it's caused by your diet, but there's always a possibility since you're lacking nutrients in other areas and your immune system would be open to a variety of things. Like AM1995, I highly recommend getting off of that diet immediately. A banana/water only diet will cause you to lose weight, yes, but not to the degree you're looking for. If you're trying a fast, no fast would last longer than 5-7 days either. Whoever, or whichever site, gave you that advice needs to be looked at immensely! If you'd like some diet advice, feel free to message me privately and I can help you with some tips and ideas that have helped me and others by my personal trainer and nutritionist.