It really depends. Some girls experience bleeding after sex for the first time, after the hymen stretches out a bit. Other than that, bleeding usually indicates some sort of injury to the vaginal canal. Too little lubrication and over-aggressive hands or penis's contribute to that.
What you can do to prevent this in the future is to invest in some water-based lubrication, like astroglide or KY. Use it liberally.
Also, allow yourself to heal before engaging in more sexual activities. Give it a few days.
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