why do I have brownish stains on my underwear after having a bowel movement

Hi!  I get this too.  Even if I wipe my butt good sometimes after a while it starts to itch and feel dirty again.  Then later when I pull my pants down to check I have brown stains in my undies.  Plus if I look in the mirror at my anus, sometimes it looks dirty again.  I'm 14 and don't know for sure but think it's probably normal for boys to get stains.  Don't worry about it but just be careful not to get in trouble for it from your mom.