Why does my penis sting after masturbation followed by urination?

I’m 19, haven’t had sex, but have had this problem for years. Sometimes (randomly, once a month or so) I get a stinging sensation in my penis that lasts for 15-45 minutes, then goes away, if I masturbate then urinate. Recently though, it’s been happening daily, and it’s very irritating. I’ve recently changed my diet – for the better I think – by eating less calories, but still the same kinds of foods. I’m still drinking water, and I’m just confused. What could this be?