I'm facing this frustrating problem and need your help!! don't know how to deal with this problem.... Tip of my penis becomes very sensitive when it's erect... if i start rubbing when mastrabating or getting oral I'll lose my erection quickly and it pains a lot after that and cannot get erection again... please let me know what to do... shall i consult a doctor?
By "tip" I assume you mean the glans (head) of your penis.
Are you circumcised? If not, can you retract your foreskin?
This is very common in uncircumcised men. The foreskin protects the nerves so they are not used to being touched. If you can retract your foreskin let it hang out for a bit, so it gets touched and becomes less painful.