Lots of bright red blood comes after stool

This has happened to me already. A lot of times. I've told my parents before but then I can't seem to tell them this time.
What kind of illness or disease do I have if
- I feel like there is like an acid in my anus
- Bright Red Blood comes out of my stool
- I am always constipated [don't poop for 2-1wk always]
- but then my stool turns out quite soft but big
- i don't drink a lot of water none at least
- pain in my butt

* I'm really scared because just yesterday my uncle passed away w/colon cancer.