Not to long ago i felt a little bump on my upper arm it feels really hard and its painless it is inside my skin?

Dermatofibroma: Dermatofibromas most often occur on the legs and arms. Once developed, they usually persist for years. They appear as firm-feeling nodules, often yellow-brown in colour, sometimes pink and sometimes quite dark, especially in dark coloured skin. If the skin over a dermatofibroma is squeezed a dimple forms, indicating tethering of the skin to the underlying fibrous tissue.

Hard bump: Could be many things but one of the most common causes for a nontender bump under the skin is a dermatofibroma or cyst.