Ah! No buying petstore rats! There is ALWAYS illness, or bad tempurments!
Now that you have one, you MUST get her a friend. Rats are so very social they really cant live alone. Try your best to make a cage or buy a cage. Make sure you quarentine in a seperate air space for 3 weeks. Dont buy her friend from the petstore. There are so many rat rescues its rediculas. Where are you located? I can help you find one. What are you feeding her? Harlan Teklad Lab blocks are great, and you can also search on google.com Sue Bees Rat Mix. Its fantastic. It also rules out having to buy from a petstore at all!
What kind of bedding are you using? How old is your rattie? I hope I helped!