DEHYDRATION!!
You say you've been drinking a lot of water ... what does that mean? Define "a lot". Your urine should be almost clear.
And if you do have kidney stones, you really need to drink a lot ... more than usual.
I've had (or perhaps still have) kidney stones, and had to go for surgery to remove one that was too large to pass. While I was waiting for surgery, I discovered that I suffered (badly) from kidney stone pain when I wasn't drinking water regularly. I ended up in hospital on morphine one day when I dashed off for my 150 km drive to university one morning without a single thing to drink ... that was actually when they discovered the kidney stones.