I noticed that the back side of the top of my penis got swollen after I masturbated last night. It's kind of worrying me. What could be going on, and what should I do about this?

Could be due to irritation or inflammation. I recommend using an over-the-counter hydrocortisone cream to reduce the swelling and discomfort. Apply a thin layer of hydrocortisone cream to the affected area twice a day for about a week. If the swelling does not improve or if you experience any pain, fever, or discharge, please consult a doctor for further evaluation and management.