She most likely about to lay eggs...boy are you in for a treat. Even if she is alone, they lay unfertilized eggs sometimes. You have some research to do. They need an area to lay them ..its recommended to use a mix of sand and dirt twice and deep as their shell. My turtle refused to use this nesting area..as most do

. I have seen a few people who have taken their turtles outside in a fenced in area and the turtle laid eggs in the yard (keep an eye on them!). They are very shy when it comes to laying eggs though. Unfortunately, she will probably lay them in the tank. This is dangerous because she may not lay them all

. Your vet can do an xray to see how many she has or you can feel for them. Theres several threads about this. Good luck! Keep us updated!