I experinced brown discharge yesterday today it is pink. what could be the cause of this?

Discharge: It probably has blood in it based on the color. What caused it - not sure as more info is needed. If you're in the middle of your cycle, chances are you had a mittleschmertz - mid cycle bleeding.

A few things: Normally a pinkish brown discharge is associated with old endometrial tissues, but cases may differ and lots of things can make this happen. Pink color is a sign of fresh shedding of the uterine lining. Brown means that during the last menses the entire uterine lining failed to make a timely exit. Possible causes:ovulation problems, puberty, birth control medicines.