Turtles will retain eggs as long as possible while they seek an appropriate nesting site. This retention can cause infection, organ damage, and even death. Symptoms of a problem may include not laying eggs, laying only a few eggs, lethargy, pacing, lack of appetite, prolapse, digging motions, and kicking of the back legs.
So, yes, you'll need to provide her with a nesting site. For a slider, that would be a 50/50 mix of sand and organic compost, about 12 inches deep. The bigger the area you can give her, the better. You may have to move her to a preformed pond or a child's wading pool in your basement to make room for everything she needs. Be sure that you continue to provide UVB light and a basking light no matter where you put her. This site has very detailed instructions for making nesting sites, including photos:
At this one, you can read more about egg-laying and egg retention:
As for when she can be taken back to her regular tank, after she lays and buries a clutch of eggs, you can try putting her back. however, you’ll need to watch for signs that she has more eggs. That would include lethargy, pacing, lack of appetite, digging motions, and kicking of the back legs. If she exhibits any of these signs, put her back in her nesting site.
This is a serious situation, so be sure to build a nesting site as soon as possible. If you have further questions, just let me know by clicking on REPLY.