Lumps can mean a few different things, but either way it means she should be looked at by a vet. A lump can be a cyst or a tumor. Depending on how young she is, it can be removed. If it is a cancerous tumor then there is not much that can be done. It could be a bug bite, but usually those are small. It can cause an infection if she has been scratching at it or biting it. It can also be an abscess which can be caused as a slight nick in the skin by scratching or an injury or even just dry skin. This can easily be treated by a round of antibiotics.
It could be because of the exercising wheel. Please avoid using stainless steel or iron wheels. Use the plastic ones and please buy the most appropriate size.