She could be getting ready to deliver. Does she look really boxy? Like she swallowed a square? If you want to try to keep as many fry as possible you can put her in a make shift breeding net. Fill a clean plastic container with tank water and put her in that. Float the container in the tank. Then remove her after she gives birth. I put lots of small holes in the plastic container, just stabbed with a kitchen knife, so water from the tank will circulate into the container. Just don't make the holes too big. You could also just make sure there are lots of hiding places for the fry and leave her in the main tank. If you have lots of plants and decor, chances are most of the fry will make it. If she is still on the bottom for more than a few days she may be sick.