It could be that she might have a swim-bladder disorder that prevents her from maintaining normal bouancy. These disorders can be genetically inherited in certain betta strains and are considered common hazards of breeding, or sometimes caused by the pressure of expanding food in the stomach against the bladder, or bacterial infections/poor water quality. A fish with such a disorder will rest more often than most fish, and either sink or float without any apparent "control" of it.
If the fish is still in good color and feeding normally, then most of them can live with such a disorder for some time. I've currently a goldfish that has been with me two years that has a swim-bladder problem. No matter how long I soak his food for, no matter what he eats, after the meal the poor guy bobs around the top of the tank like a top unable to submerge. This will usually last for 24hrs until he's digested all his food. I don't feed him again until he displays neutral bouyancy and normal swimming patterns. I do not belive it is bacterial/water quality related because his tankmates are fine, he's in good color, and well, he's still alive.
As for your female, if she is in good color and feeding normally I would allow her plenty of places to rest and make sure her water quality is top notch. I would not breed her though. If she does carry a genetic defect she may pass it to her fry and only perpetuate the deformity within your lines. Swimming will exhaust such a fish more rapidly than a fish with normal bouyancy control, so if she stays around the bottom, that is good for her.
If you are feeding a dry type of pellet, try soaking the pellet before you feed. This will allow it to absorb water and expand outside the betta's gut. When it does this inside the fish, the expanding food places pressure on internal organs and could cause swim-bladder problems. Also try feeding more softer foods, such as microworms or tubifex worms. Bloodworms have a hard shell that might irritate the problem more.
~~Colesea
