Ireneo is correct. Take her advice.
Hamsters can get sebaceous cysts, traumatic injuries from their cage's or from other hamsters when caged together, skin parasites, other infectious skin diseases, and skin tumors. Skin tumors are more common in older hamsters, however.
You could take it to a vet or return it to the pet store. They should be notified that their hamsters may have a problem, check to see if it's something contagious.