Mark, best thing to do is to see how this develops - it may be simply connected to the masturbation, particularly if you did anything different, such as if you were erect for much longer than usual, or you applied much more pressure than usual.
Men who use penis developers can get things like this. If you can, do not masturbate for 8 to 10 days - but if you cannot hold off that long, make sure not to use pressure, do not stay erect for long, so, make yourself finish within 10 to 15 minutes, or less if this is more time than your regular.