Can condoms be used for masturbation? Or is that a waste?

Should a normal man's penis become erect for a certain number of times throughout one day (without any external stimulus)? Or, do these erections occur only while a man is sleeping? I have been masturbating once every day or every other day, and I have noticed that I rarely get any of those spontaneous erections during the day at all. Plus, I have been waking up with no morning erection during the past week. Have I been masturbating too much?

What is oxytocin and does your body produce it during masturbation? I heard you can buy it, but if you do, does that mean you are going to start producing milk at random times for women?

Sometimes I masturbate before I go to sleep at night. Usually nothing is wrong, but sometimes (and this always occurs at night, I've found) after I masturbate, a few minutes later I have to urinate. And when I do, it is usually very painful. The feeling doesn't pass either. My scrotum usually gets very tightened up, and there is painful feeling above my crotch. Usually I just sit there, drink some water until the feeling goes away. ANY idea what this is?