A possible cause of a lump forming underneath a scab is infection of the wound, according to Drugs.com. Sometimes the skin over the wound may heal while an infection develops underneath, reports WebMD.
Infection of a wound can be caused by bacteria entering a break in the skin. This bacteria can come from the skin itself, the environment or the puncturing object, explains WebMD. When bacteria enter the wound, an infection can develop underneath the skin, even after the skin has healed. This causes a lump to form underneath, called an abscess, which is essentially a pocket of pus.
Additional signs of wound infection include fever, a swollen and painful wound, blood or pus coming from the wound, a foul odor coming from the wound, and possibly dizziness, reports Drugs.com. Treatment options include cleaning the wound with soap and water to wash away germs and rinsing the wound with sterile water. Antibiotics may also be used to help fight the infection.
If the wound becomes severely painful, is bleeding through any applied bandages, the skin around the wound is numb, the individual is unable move a limb below the wound area, blisters develop, or the skin peels or changes color, immediate care should be taken, reports Drugs.com.