What could be a small lump under the skin on chest?

Many possibilities: If red & painful, we worry about abscess (pus pocket) or ingrown hair or pimple or infected sebaceous cyst. If not red and not painful, then could be uninfected sebaceous cyst or lipoma (fatty tissue), possibly even lymph node (don't know where on chest you're referring) as long as it is freely mobile and not fixed/hard/matted down. Best to check w/your family doc for evaluation.