Question says it all.
Most Helpful Opinion
If a girl is hornier on her period it is entirely mental. Biologically that makes zero sense. Hormone levels peak in the middle of your cycle when you ovulate two weeks before your periods starts. Unless something else is going on, the days right around ovulation are when a girl is naturally the horniest. The body also responds to this hormonal spike by making the vagina wetter to be more receptive for sex.
If you are on the pill all of the above would not apply because there is no hormonal spike because the pill mimics the hormonal levels of pregnancy (especially progesterone) preventing an estrogen spike and preventing ovulation.