Sometimes one of my balls gets into the body, up in...then it hurts when i do sex. how can i prevent this. ?

Anatomical: You might try looser underwear, but some people have a shorter gubernaculum testis - than others - this is a ligament that holds the testis like a tether.... If the pain get severe or redness /tenderness occur - you should definitely see your doctor - sometimes the testes can tortion or twist and become very painful - and this is a serious medical issue.. Good question - hope this helps!

See a urologist: You probably have a retractile testicle. This may require out patient surgery. See a urologist for a physical exam.