Agreed. Blood in your semen isnt the end of the world, and it is, indeed, fairly normal--especially among younger men. It usually occurs due to an injury near on the inside, near the very tip of your penis--most likely from rough handling, as noted by Jeff.
2 two solutions:
Either dont be so rough while jerking off/having sex, or continue the way you have been, and your penis will naturally compensate in the areas where injury is most likely to occur. (and no, solution number 2 *isnt* a bad thing)