These are signs that you're growing up and becoming a mature adult. Sexual thoughts and feelings are a normal part of being an adult. However, you are in charge of your own behavior. Thoughts and behaviors are two different things. Most people have thoughts about different sexual experiences, but may choose to act on only some of them or none of them.
Responsible adults consider the consequences to themselves and others before acting sexually.
It is very common for girls to notice an increase in their sexual desires, thoughts and fantasies around the time they begin menstruation. In fact, it usually increases to from eight to 10 times the amount found in a girl's body during childhood. This is what causes the normal discharge that you have seen on your underwear. It's part of the natural cleansing process of the vagina. In addition, sometimes lubrication of the vagina results from a thought or fantasy that is related to sex.