What happens when my muscle on the front side of the bottom half of my leg has become a bump and hurts?

Muscle fascia hernia: Sounds like you may have a fascial hernia that allows the muscle to locally herniate through due to internal pressure. With activity, the muscle pressure increases and the visual bump enlarges as more muscle extrudes through the defect. Exercise induced exertional compartment syndrome may also lead to this injury pattern. Orthopaedci surgeon evaluation is indicated.