Sharp Pain While Ejaculation In Lying Down Position

Patient: Hi,im having a problem with my ejaculations,i usually masturbate while sitting down,always did like that for several years.now recently i started masturbating while laying on my back and its causing a sharp pain while im ejaculating,,like moments before ejaculation and during ejaculation ,i will have a sharp sting,pain that will stop when i finish ejaculating.it only happens when im laying on my back.is this a problem,should i visit a doctor,?or it is simply because its harder or that im not used to it?im 18 years old,and i have depression problems.

Doctor: