This article says that a penis can become numb if it is injured. Maybe you did that when masturbating, don't know. Here's the article https://www.medicalnewstoday.com/articles/322694.php. The take home message though is that if it is unexplained, to see your doctor. And while you are there, talk to your doctor about anxiety and diarrhea every day. Anxiety is a treatable condition and maybe the penis problem is connected as you know a lot of it is mental. So, go to the doctor.