| There really is no way to tell. It could be anything from a simple pimple or boil all the way to a neuroblastoma or infection of the maxillary sinus. It could also be a manifestation of any number of skin diseases. Whats more, it could be a skin cancer, such as a basal cell or squamous cell carcinoma.
With that bare a description, without a picture, without knowing about any associated symtoms (is it painful? how long has it been growing? have you had any weighloss, headaches? do you have any similar lesions?) there really is no way to tell.
I would most heartily suggest that if you are concerned about it you should go to a doctor. Either a skin doctor (dermatologist) or a primary care doctor (internist, family practicioner, etc)
Hopefully, its just a pimple, but you really should make sure, as things are easier to treat the earlier they are caught. | 
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