Sick Betta Fish help?

I have had my female Betta for several months. I have her in a one gallon bowl and feed her several times a week a Betta blend by Tetra. I change her water weekly and use a water cleanser every time. About two or three weeks ago she started disappearing into her castle and stays there for two or three days. She is refusing to eat, and when I have seen her out she only comes up for a little air, spending most of her time on the bottom or under her castle. She is in an active room with windows nearby. She seems a little bloated, but nothing excessive, she has not changed color, her eyes are normal, there is no fin rot or scale problems. She does have a little trail of white stuff like a thread coming from her underside, I'm not sure what it is (doesn't look like poo). She just seems depressed, help :(