I suddenly discovered a lump on the back of my head. What could this be from?

Trichilemmal Cyst. Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population.Greater than 90% occur on the scalp, where trichilemmal cysts are the most common cutaneous cyst. Trichilemmal cysts are almost always benign. Treatment is surgical excision.