Today is an interesting day. An enormous low pressure weather front is moving across the country, slowly, in waves. The air pressure is dropping rapidly all over. This can cause sinus pressure - not nose, but in the air filled pockets of your skull. http://i35.photobucket.com/albums/d168/peplreallysuck/sinus.jpg
These ones. Im sure you'll notice they are exactly where the pain is.
Now this pain isnt directly causing the dizzyness, but the pressure is. Its Vertigo. If you were ever wondering what vertigo feels like, now you know. This is because the same pressure change is pressing on your ears, creating positional vertigo, since your inner ears regulate balance. So when you change positions fast, it all takes slightly longer than normal to realign properly.
The pressure you feel when you change positions just confirms it. The shower thing too, its almost universally a relief, however temporary, of the symtpoms.
Today sucks. Ive had the exact same thing as you all day. Im just much more familiar with it. Even so its still kinda scary. But just try not to stand up too quickly and you should be fine.