My daughter just had her first menses at 7. What do I do?

Source of bleeding? Is this blood only on toilet paper? How long did it last? Did you see from where it was coming? Constipation? Any other signs of puberty like breasts? Has she had new adult exposures? A period is the least likely cause of this type of bleeding, though still possible. Anal fissures from constipation, aggressive wiping, and sex abuse have to be considered, especially if no breast dev. Please see doc.
Early menses. If she has other signs of puberty ie breasts and pubic hair it is likely due to precocious puberty. See a board-certified pediatric endocrinologist asap. If there are no other signs of puberty it may be due to an ovarian cyst that ruptured. Have your regular doctor order an ultrasound of her pelvis.
Too early. That is abnormally young. Your pediatrician needs to refer her to a pediatric endocrinologist for tests.