I would think that it wouldn't be the actual ejaculation that hurt, but it would be the time right before when your testicles are put under a significant amount of stress by your scrotum and surrounding muscles (I'm assuming that you had an incision on yours scrotum during the hydrocelectomy). I would suggest careful masturbation... even with your girlfriend at first. That way, you control what happens to you and you can gage what hurts and what doesn't.