Why doe my pubes itch after shaving>

It could be "razor burn" typcially developed by using a razor without shaving cream and/or going against the grain of the hair.

I am a guy so my answer is not going to be as relyable as any women's.
It is completely natural for you to feel an itch down there after shaving, it depends on whether you ended up using just a sraight razor. It helps to use shaving cream to reduce any chance of iritation, also when done shaving it helps to apply lotion to help soothe the area. You may still feel a slight itch or iritation and it may feel weird to wear underwear for the next couple of days. My only suggestion is just to let the area air out by not wearing underwear to bet and the senstaion will go away after a while.

My good friend heaven knows much more about this subject and has a video on it where she gives advice about shaving in that area. Here is the link, watch it and I hope it helps.
Shaving...girls only :-) video FunAdvice.com (http://www.funadvice.com/photos/view/164167)

I also had the same problem,
I learned from someone I know to put arm deoderant on that area. Don't put any inside of the vagina of course lol. But if you put it where the itching is, it usually helps a lot. :)
I hope this trick works for you like it worked for me.

If you really feel the need to be hairless, make sure you are using a new blade every time. And maybe shave cream specifically for sensitive skin. Or you can try waxing. It's painful but only while you're having it done. Personally, I prefer nair's bikini area hair remover cream. It's effective, pain free, and you won't get razor burn.