Brown streaks in underwear????

Okay this is really bothering me . & its been going on for months now.. Im a very clean person and i cant understand why i keep getting these brown skid marks in my underwear after a couple hours throughout the day. When i wipe myself its just urine but on my underwear it leaves this..... no funky smell . When i wear black underwear it looks like the vaginal area of my undies has been bleached . im trying not to freak out but if someone has been through this please let me know ???