What causes a period to happen twice in one month while being sexually active but pull out method was used?

Many possibilities: You may have a hormone imbalance or something else causing your period to come twice a month. It has nothing to do with your sexual activity. See your doctor to sort this out.

Unreliable method: "pull out" method is unreliable because there are sperm in pre-ejaculation fluid. You may want to check a home pregnancy test, as the second "period" may not have been what you think. See your doctor if this seems confusing. Good luck.