1/2 of my rats face is swollen

Question
HELP!  I came home today to find sweet Bella with 1/2 of her face swollen!  I can't find any lumps that would indicate tumors and have no clue what's going on.  She's normally very active and acted as if she wanted to go exploring as normal except her right eye is so swollen it's almost shut.  The swelling extends from the right side of her face into the top part of her head.  
What can it be?
We clean her cage 2x per week.  Bought her at a pet store in March.  She's in the cage with another female rat (Alice)who appears to be normal.
We just introduced them to yogurt chips (which she loves!) but she only gets 1 or 2 a day.  Nothing extreme. Could this be an allergic reaction?
Her bedding is the soft cottony bedding sold at pet stores for rats.  It's changed 2x per week.  We've been using this same brand since we first got her.  That's not changed any.

Answer
It could be an allergic reaction; rats are normally most prone to reactions to nuts and dairy products. It could be an abscess as well that sprung up over night. The only way to be sure with it being in that location is a trip to the vet. The vet may know just by looking at her, or they may try a course of action to rule one or the other out first. I'm sorry; good luck, I wish I could help more!