Why do some women get horny while on their periods?

Simply hormonal changes drive changes in a persons sex drive throughout their menstrual cycle. Hormonal changes also drive neurological changes that see womens senses being increased during menstruation, and the brain can also incorporate increased vaginal lubrication from menstrual flow as increased lubrication when aroused. There are also some cultural factors such as the idea of sex during menstruation being a little taboo, which can drive increased arousal for some.

Its the hormonal imbalance that is associated with your menstrual cycle. I know I get horny when I get near my period and I did a little research and what I remember about it was that because of the change in hormones to trigger the actual period causes testosterone to be at higher levels. Don't worry...women have testosterone and men have estrogen, just in lower amounts. That is part of the reason guys have such consistent sex drive, the higher levels of testosterone. But that is another answer to another question. So although its not shown in all women, its normal.