This is a little hard to ask. [NSFW] Anyone who knows anything about this should comment, please!

Meaning that it's hard to ask because I don't know how to explain it, and I'd love to know more about it. When I have sexy alone time, (female, by the way) I use it with Clit vibrators. Most amazing pleasure ever. But, After a while I start to build up a bunch of pressure. After little time, my body starts to twitch and shake, and tighten up. If I continue, It's hard to breathe because with every breathe I take my lungs seem to stay sucked in. (Wording is bad, I'm so sorry.) By now I can't really move myself, so I stop. Afterwards there is some pain in whatever part of me was tightening, and anything that hit something when i was shaking hurts as well. I hope this is a well enough discribtion, because I have no idea if this is normal, because in porn I've seen nothing happens like this. Please help by telling me anything you know!