I am a 16-year-old male who masturbates 1 or 2 times a day. During some of my sessions my feet – the right, left, or both – start to cramp up and ruin my pleasure.

Do you know what's happening and how I can correct it?

It sounds like it might be your calf muscles, perhaps because you have a habit of pointing your toes while you masturbate. Whatever the case, you're strongly working some muscle that doesn't ordinarily get worked. (No, not that one!) When you start to feel a cramp, try to figure out what muscle or muscles are contracting, and then contract the muscle that works against it until the cramp stops. For instance, if it's the muscle at the back of your calf, stop pointing your toes and draw your foot up instead, and hold it there. Then make a conscious effort to stay at least partly in that position while you masturbate. Eventually you'll "kick" the habit and won't cramp up.