Is it normal that I always have stains in my underwear?

Whatever you do, don't wear thongs! Maybe you have hemorrhoids? Get plenty of fiber in your diet to prevent constipation. Constipation can make hemorrhoids worse thereby causing increased inflammation in anal area which makes it harder clean when you wipe. Then later when the swelling goes down and you start to sweat your ass will itch and if you scratch it *poof* you get those dreaded skid marks!