I had the same thing happen to me at one point. When I went to the doctor I was told that nothing was wrong as long as this wasn't a chronic problem. It seems that sometimes things get mixed in with semen (for example: sometimes semen is more yellow than normal, but this only means there's urine in it). If you get blood in your semen the majority of the time I would recommend you get this checked out. Otherwise, I doubt that anything is wrong.
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