Penis extensers may often cause injury to the penis or nerve compression which may cause such symptoms and hence is not advocated. There have been techniques which are used to increase the size of the penis like jelqing, penis stretching exercises as stretching makes cells composing your penis grow increasing the length of your penis and kegel exercises which target the pubococcygeus muscle and thus helps in maintaining the erection for a longer time. You should go ahead with these exercises only after a consultation with your urologist as they have their own side effects.
I hope it helps. Take care and regards.