I Read That It Was Good To Drink 3 Litres

I read that it was good to drink 3 litres of water a day, so I tried it. I've peed a lot today due to the amount I've been drinking, but then - at about 21:30 - I went to the toilet again and peed out blood. I stayed on the toilet to ensure that blood was coming out in my pee, not just out of my vagina, and every time a little bit of pee came out, I felt a horrible pain in my bladder. I've never had anything like this before. I don't want to tell my mum because I would find it embarrassing, and I definitely don't want to have to see a doctor. Please help, I'm only 14 years old and I don't know what to do. Thank you.