Omg that happened to me too!! I agree with this comment on top but Dont masturbate in a while..!
It's not your hymen, because if you've been masturbating for a while it would have broken a while ago. It could just be irritation or a cut inside your vagina that happened while you were do this. Do you have any other symptoms? Another possibility would be an infection, though typically you would have itching, redness, and/or a different type of discharge from what you normally get. It does just sound like irritation though, so it should be fine and heal on it's own. Just leave the area alone, don't douche or try to wash the area with anything other than water, and let your body heal. Take care.