Why do my testicles disapear when fully erect or ejaculating

This doesnt happen all the time maybe 30% of the time when i masturbate with lubricant or have sex, my testicles dissapear into the top of my scrotum. Basically in-line with my penis. I can still see them but it seems that they get sucked in when im at climax (or pushing). I dont know if its a hernia or something. After i ejaculate i have to gently use my fingers to guide them back down into my scrotum. Does anyone hav this kind of thing???? What is it and why does it happen, its so annoying! I havnt seen a doctor because at anytime of the day my testicles look perfectly normal.