It could be and that is the most likely explanation. Do you have any ropy, swollen veins near your testicles? They are called varicoceles if you do and they can ache if you masturbate, particularly if you do it more than once a day. You shold see a doctor if you do have them just to make sure they are not too serious. They can affect fertility. If you don't have them then my best guess is the first explanation. Has it always happened or is this just a recent phenomenon? If it is recent it could indicate an infection or some other problem and you should see a doctor.
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My testicles hurt 
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