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Just pull the skin back and see if you have injured anything on the foreskin or glans.
If there is no injury then the most common cause is hematospermia i.e. blood in semen. The most common cause for the same is high blood pressure. You should get your blood pressure checked and beyond that nothing much needs to be done. Hematospermia is caused by high blood pressure in 70% of patients and no cause is identified in the rest. It settles down by the control of hypertension ( if that is the cause) or just by itself in 4-6 weeks.