If by saying "my ball sack has gone dry", you mean you have little or no semen coming out of your penis when masturbating, then you may want to give it a rest and masturbate just once a day or less until you see some improvement.
They'll know what to do if you get a hard-on. It's normal and just shows you're a normal guy. It likely has nothing to do with your jerking off. More likely it's skin drying from the winter season, from your body changing normally, or possibly from something you eat. Rub a bit of body moisturizing lotion on your scrotum [sac] and see if that makes a difference.