The simplest answer could be either the exercise wheel or the cage itself (if the bottom is wire). Try taking the wheel out for a day or two and see if there's improvement. Make sure the bedding of the cage is suitable to cover the whole bottom as well. (Are your hamster's nails being trimmed properly, or kept to a healthy length by other means?)
What type of hamster is this?
It could also be the start of a bacterial infection, or something more serious. I'd take him to the vet if you try the first options and don't see any improvement, or worse, the condition spreads.
Copyright 1994-2016MedHelp International.All rights reserved. MedHelp is a division of Aptus Health.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. Med Help International, Inc. is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.