Hard lump on arm

The size of the lump and the speed of growth might have been helpful. If the lump is very small (up to 0.5 cm), then you might be experiencing blocked skin gland. It might be accompanied with redness in the surrounding skin.
If the lump is bigger, you might be experiencing lymph node due to inflammation close to that area or a tumor. If it turns out to be tumor, it does not necessarily means that it is malignant.
You might want to see your family doctor. He might recognize it as enlarged lymph node if he finds the primal focus of the inflammation or might refer you for biopsy of the lump to check whether it is a tumor.