How can I have UTI symptoms even though I drink lots of water?

I am a 61 year old widow.  Had a large kidney stone removed a year ago and have been diligent to drink nearly a gallon of water every day.  I have felt great and had no problems, but just recently started having UTI symptoms.  Frequency (but that's nothing new since I've been drinking lots of water), pressure when urinating and slight pain.  And last night I had to get up every hour to urinate and when I got back to bed, I felt like I still had to go.  My urine appears clear and no blood.   What is going on??