nerve damage can be the cause of loss of sensation, and the way you masturbate can also be unhealthy for your penis, get an exam and be sure to tell the urologist ALL of your issues even if you think they are unimportant.
have you ever been to an urologist? have to ever had your feet slip off the pedals of your bike and land on the bar? you could have an injury , in either case i would sincerely encourage your to at least make an appointment.