I think you should get checked out by your doctor. They'll be able to examine and diagnose it. It's under the skin, correct? Is it hard or is it moveable? Could it be a cyst? They don't often appear on the penis but it can happen. I'm assuming this isn't like a raised bump but you are saying lump so under the skin. Lymphceles occur after sex or masturbation so perhaps this is the reason. It's pretty harmless and a blockage in the lymph channel. Here's a list of possibilities but lymphocele seems like it could fit. https://www.healthline.com/health/mens-health/lump-on-penis#lymphoceles