Doesn't sound like retrograde ejaculation
Retrograde ejaculation occurs when the semen that is supposed to come out the end of your penis during ejaculation "goes the wrong way" and ends up is your bladder.
It is fairly uncommon but it is usually caused by surgical damage (i.e. prostatectomy), medications (mostly psych or blood pressure medications), or nerve damage (i.e. from diabetes).
It is generally painless. The only thing most people notice is that a decreased amount of semen is produced during ejaculation.
You'll pee out the semen the next time you piss. It's basically harmless. The only medical problem is that it's harder to get someone pregnant if your semen is going the wrong direction.
I've never heard of it causing appendicitis like pain or pain with urination. I hate to second-guess your ER doc but the diagnosis sounds a little suspicious to me.