A cuter, cuddlier relative of rats and mice, the hamster is a popular pet for its small size, its gentle demeanour and the relatively low maintenance it requires. If your hamster becomes bloated, it could be due to any number of reasons, ranging from relatively minor to life-threatening. Whatever the cause, you should contact your veterinarian immediately if you fear for your pet's life.
As is the case with humans and many other animals, one common cause of bloating in hamsters is constipation. Common signs include minimal amounts of faeces, visible straining during defecation or attempted defecation and a "hunched appearance" when moving around the cage. Hammy's World recommends that you increase your hamster's intake of fruits, vegetables and water, noting that "constipation is usually easier to prevent than to treat."
If your hamster is bloated and your attempts to reduce constipation are ineffective, there's likely a deeper cause. Broadly defined as "fluid-filled cavities or sacs," cysts (particularly internal ones) are a conceivable cause of abdominal bloating in hamsters. If you believe your hamster may have a cyst, it's important to take it to your veterinarian. Tumours, cancerous and otherwise, can sometimes be mistaken for cysts.
According to Hilltop Animal Hospital, amyloidosis, a condition that can develop with old age, occurs when protein builds up internally, most commonly affecting the kidneys and the liver. Abdominal swelling and/or bloating is a typical side effect.
Secondary symptoms of amyloidosis may include difficulty urinating, loss of appetite and poor coat quality. If your hamster is in fact suffering from this condition, you may be limited in the action you can take--the hospital goes on to characterise amyloidosis as "eventually terminal."