I'm 18 years old, my scrotum shrinks and becomes...

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What we advice you ?

scrotal sac can get shrinkage if it's exposed to cold whether or air condition and expand more upon hot whether and high degree but you must check for not getting testicular atrophy so you must do 
1.scrotal ultrasound to look for testicular size and shape and if the blood vessels reach it 
2.seminal fluied analysies 
3.hormonal measurements as andrgen testosterone prolactine 
if you sure that the above mentioned is okay then there is nothing to worry about and it's normal phenomnen for tempreture change