Ejaculation problem. what's wrong?

I'm 17 years old. I masturbate a lot but lately i've noticed a strange symptom. Before I'm about to , I get these strange contractions. Similar to the ones that happen during ejaculation. I don't know how to explain it. I've never seen anything about it before. Does anyone know what it could be? I was recently diagnosed with epidimytis but I don't konw if it has anything to do with it. Normally I would become more stimulated progressivly during. But now it happens all at once. It's so hard to explain. It could be a problem with my boulbospongiosus muscle. You're supposed to contract when you squeeze down on the glans, but I don't. Does anyone know what this could be related to. I plan to go to the urologist but I m afraid he won't help me.