What is wrong with my mouse?

Question
Hello.  I have had 2 pet mice for a couple of months and I left state for 2 days, when I came back, one of my mice had/has a puffy face and it's very red where the area is.  Could this be mites?  If it is, how do I get rid of them?  What other sicknesses might it be?  Or could it be the shavings??  

Also, I introduced a baby mouse into their cage today, my one mouse, who is the gentlest mouse ever, invited her in with no problems, the other one (who has the puffy cheek) has been following her around sniffing her rear and front excessively, but not hurting...Is this just an aggression thing, or a getting to know her thing?  The mouse with the puffy cheek is acting extremely HYPER and I am wondering if it's due to the new house mate, or the puffy cheek?

Thank you for your time!

Answer
Katie, If you can, check the inside of her mouth.  It sounds like it could be an infected tooth or and injury that has abscessed.  If it is an infection, she is probably running a fever which will make her irritable.  Is she eating and drinking normally?  I would have her checked out by a Vet and possibly get her on antibiotics.  If it were mites, she would be itching all over.

You didn't say whether your older mice are male or female.  Remember, mice can reproduce at a very young age.  The sniffing could be a territorial thing or a hormonal thing or just irritability because she doesn't feel well.