Is my pet rat depressed or sick?

Question
Yesterday I went to Petsmart and bought a sweet, 3-month old female rattie friend(named her Summer) for my 1-year old female rat(Roxi).  Roxi has been without a cagemate for months, so I decided to go ahead and get her one since she was getting quite lonely.  

Summer had two sisters in her cage at Petsmart.  She sneezed a few times when I was holding her in Petsmart, but I didn't think anything of it.  I introduced the two in the car in my lap, and Roxi LOVED her. Pushing her around with her nose, snuggling her, sniffing her, grooming her.. so cute!!

When I took the two home, I decided since they already got along so well I wouldn't have a problem with Roxi being defensive or mean to Summer in the cage, so I let them both in, one-at-a-time. (The cage is tall and wide, with three levels and a hidingplace underneath).  Roxi explained her dominance by climbing ontop of her (appeared to be trying to mate with her) and Summer let her.

Summer has been sitting in corners of platforms in the cage, with her sides pressed up so much against the side of the cage that her arm and tail dangles out.  Her eyes are half-closed and she won't move much when I nudge her.  Roxi comes by her every once in a while and gives her a very thorough grooming, but Summer doesnt groom back. Only lies there and occassionally grinds her teeth happily or squeaks when Roxi loves on her.

Summer has these fits of sneezing (like at Petsmart, but on and on).  Sometimes it goes longer than others, sometimes it's just a sneeze and she's done sneezing.  But frankly it worries me sick.  I would hate for her to have a respiratory disease! But some people have told me this is absolutely normal and she is just very sad right now from leaving her cagemates at Petsmart..

When I woke up this morning to check on her, there was poo all around her.  Does she not have the energy to get up and go to the bedding or what? I cleaned it up..
Occasionally I will see her eating or grooming herself, and when I take her out she walks around and cuddles with me..

Is she depressed or sick?  And if so, do you know what kind of sick she is and how I can make her feel better?

A fast response is appreciated! Thanks!

~Mae

Answer
She is sick.....and I am sorry about not having a fast enough response. I had been out of town and did not want to remove my name for VACATION since there is nobody else here that has the expertise that I have to help...or at least the last time I checked there wasnt.

Anyhow, there are a few things here I want to address. One, do not listen to "people" unless they have alot of education about pet rats. They offered you advice but not the right advice. You should have been instructed about proper quarantine. All good websites about rat care should have information on quarantine.  This needs to be done to all new rats, no matter where you get them from. They need to be kept in another cage in a different room where they dont share the same airspace. Three weeks is the length of time needed to allow for any viruses to incubate and two weeks for any eggs from endo or ecto parasites to hatch.  After the quarantine, a slow introduction is needed, with the rats first living side by side and being allowed to mingle through the cage bars. After a week or so, unless of course they seem to be very accepting of each other, letting them meet face to face without the cage bars being in their way is a good idea to  do. It seems that all is well with them and they met each other just fine which is not a real shocker since it seems to be alot easier for females to get along than males.  

Do check out my  website, sandyscrittercity.com and check over the info on common rat illnesses etc...  and get back to me right away.  ALso where are you located?  You need a vet for her possibly and not any vet will do.