Could be a fissure or a hemorrhoid. What to do will depend on where the blood is coming from. If it is towards the opening, applying some vasaline on and a little inside your anus for a while should clear things up. If it is deeper, you may need a doctors help because once that tissue has been cut, it tends to open up again each time you have a bowel movement. A doctor will prescribe you with something that can help with that. If you feel a hemorrhoid (swollen tissue on a part of your anus that leads into the bowel) then buy some preparation H to get the swelling down.
Don't be embarrassed. This is no big deal for a doctor. They see it all the time. If you are worried, go and see one. The cause of your problem has not ben established but it is a good idea not to sit on anything hard like cement and especially cold.
Sounds like hemorrhoids, but I would check with the doctor anyway (It is your life after all).
It could just be something dodgy you have eaten, some stuff you eact does not digest well and can cause some iritation on evacuation, there is also a chance that it may be a hemriode, what I would suggest is give it a day or two and if you still have the irritaion and the blood spoting go in for a examination, while you may feel uncomfortable about it rest easy, they are professionals who have seen it all and done it all, they will make you feel at ease and will help you much more than anyone here can :)
Best of luck.