Puffy cheeks

Question
I have a two and a half year old rat, Noel, with puffy cheeks. She has one benign mammary tumor but other than that she's has healthy as can be. She is still very active and doesn't act like she's in pain, she's also still eating and drinking. When I noticed it today I got out my younger rat who is over a year old and her cheeks were normal. I currently have three rats but Noel lives alone in a one level cage due to bullying by the younger rats and clumsiness in her old multi-level age.
I've looked all over the Internet and all I can find is information about abscesses and such. I don't think it's an abscess because both her cheeks are puffy and she's not acting painful. I also haven't changed her diet so I don't think it's an allergic reaction.
I have had five rats in total and never seen anything like this. Please help

Answer
Hi Faith, so the puffy cheeks appeared suddenly?  A photo of Noel, before and after, would be helpful if you can send one.

Just so you are aware, rats inherently mask their pain very well unless the pain is unbearable.  Rats will also continue to eat and drink unless they are extremely ill.  I had a rat recently who was close to death.  She had numerous cancerous tumors inside her and she was hardly able to move, yet she continued to eat and drink until I had her euthanized.  

It is possible Noel has malocclusion, which is a condition where teeth curl into the cheeks causing infection, swelling, and pain.  Allergic reaction is also possible, tho you say her diet hasn't changed.   

Take a look at her incisors(4 front teeth) and make sure they are visible and straight as they should be.  Also, smell her mouth/breath for a foul odor, which is a sure sign of infection.

Or it might be neither of these.  Regardless, a visit to your vet is imperative!  You should use a vet that specifically has experience in treating rats.  If you need help finding one near you, write back with your city and state or the closest large city.

If there is infection, she will need antibiotics and pain meds prescribed by the vet.  If she has malocclusion, then she will need to have her curled incisors trimmed biweekly under sedation by the vet.  

I hope that it isn't anything more serious than this and that Noel recovers quickly with treatment.  Please write back and let me know how she's doing.  If I can help you with anything else, don't hesitate to ask.