Hello and thanks for researching this very important question!
Hamsters can get foreign matter (bedding, sharp seeds, etc) stuck in their cheek pouches.
This, along with dental infections, can cause swelling and pain.
If you do not find anything on inspection of the pouch and part of the mouth you can see, I do recommend that you have her examined by a veterinarian who is familiar with Pocket Pet medicine (not all dog-cat techniques and medicines are safe for hamsters, so you want assistance that is aware of this!). There may be something lodged far back in the mouth/throat that you can't easily reach, or she may have an infection.
You can use these links to locate qualified medical help:
http://www.cavyinfo.com/html/vet.htm
http://www.aracnet.com/cgi-usr/seagull/vetfinder.cgi
http://cavyspirit.com/vets.htm
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