I i think it might be a tumor but im not a vet so i looked it up....
Lumps
There are a number of types of lump your hamster could develop. Some are fatal and cause severe symptoms and some are relatively mild. A lump may be a cyst or blocked gland or it can be a fatty mass or cancerous growth. Most small surface lumps can now be fairly safely removed by a competent vet if caught early. Some lumps aren’t dangerous and may not need removal.
The most common site for lumps is the cheeks and neck but they may occur almost anywhere. Adult females especially nursing mothers have a risk of developing a mastitis. The area around a nipple and breast may become red, hard, and swollen. The hamster should see a vet for medication and in extreme cases surgery. If the mother has nursing pups and they are older than a week, carefully take the whole cage to the vet. If the pups are younger, I would probably wait to take her. Scent glands can also become infected and swollen. Dwarves have a scent gland on their bellies about where their naval should be. Syrians have scent glands on their hips, the outside of their lower waist and thighs. Young pups are most susceptible to this type of infection.
Internal lumps can be more difficult. Most internal lumps are usually inoperable mainly because the hamster can not survive long enough under general anesthesia. Then too, there are conditions under which you may never see the lump until your hamsters passes away. The best advice I could give here is play with your hamsters routinely; pay attention to the way they look and feel. Every time I clean a cage the hamsters in that cage get a physical exam. If I find a lump, which thankfully is very rare, the hamster goes to the vet to have it checked.
Remember anytime you have any trouble with your hamster a good veterinarian can be you and your hamster’s best friend. I try to learn something new every time I visit a vet. Since I’m paying for the visit, I expect the vet to communicate with me. You should too.
that info is from http://www.geocities.com/calhamassoc/HamsterHealth2.html
heres more info on hamsters having lumps
Hamster illnesses and diseases - skin problems
Lumps and bumps - if you discover a lump on your hamster don't just leap to the conclusion that it's cancer (it could be), more often than not, it could be an abscess. Abscesses are usually caused by bite wounds from fighting. These often form hard painful lumps under the skin where the puncture wound has closed up. Hamsters can also get abscesses in their cheek pouches - these would've resulted from puncture wounds from food or bedding materials. If you suspect an abscess in your hamster, you will need to take it into the vet to have it drained and antibiotic therapy started.
i got this info from http://www.ask-the-vet.com/hamster-illnesses-diseases.htm
and heres a little more info ion hamsters having lumps
Parasites
Symptoms Scratching. Possible malaise. Red patches on the skin, flakey skin. Spots and small wounds.
Causes A variety of organisms can live on your hamster. Ticks, lice and mites.
Treatment Firstly, higher parasites (the ones you can see) are unlikely to kill your hamster as they will be killing themselves by killing their food. If your hamster does die, there are either a very large number of parasites living on it or it was weak or it died of an unrelated cause.
However, noone likes parasites. Firstly, if they are skin parasites there are plenty of shampoos and lotions to kill parasites for pets. ONLY use medication that says you can use it on hamsters or birds, everything else will be too strong and WILL kill your hamster. After washing it in the lotion/shampoo then rinse thoroughly to prevent them licking any off, then put in a completely clean cage i.e. nothing that hasn't been cleaned is given back to the hamster. eParasites can come in via bedding, sawdust, food and other pets. Be sure that the cage is completely clean. Secondly, medication from the vets can be used that tends to be given orally.Medication Small animal sprays such as Johnson's Anti-mite & insect spray for cage birds.
i got this info from http://homepage.ntlworld.com/wrath.voti/ill.htm
so it could be cancer, parasites or just a wound from itiching. eaither way i woul see a vet just to be sure i would hate for your hamster to die.