Help, Newbie worried about Gecko

Hello my name is Jennifer I have a 9 year old son. During the summer we bought a Leopard Gecko from a Convention during the summer. We have had her about 6 months, she is a full grown adult about a 1 yr old. About a week ago she stopped eating food I cleaned the tank on Wednesday night to see if she is pooping and nothing, today I went and got some Vitamin B-3 drops with electrolytes hoping it will help. Anyway I was just watching her and when she walks she goes very slow her back legs are spread apart and she stops every few steps, I noticed her dig and then put her back end over the area she dug and just stand there then she went and hid in her rock. I am not sure what to do so I got onto the intranet found this site and an entry with similar problems but my Gecko has not been with a male, I don't know what to do and I am worried she will get weak and sick. Does anyone have advice for me. Let me know any help appreciated.