I had the same thing with my last pregnancy, c.shallow.
Once the liver test had been ruled out, I was informed it was hormonal changes. It was bloody annoying, and lasted until little Pippa was born..the relief was tremendous! I tried cool cabbage leaves on my belly, which eased it a little..as did cool baths.
As a side note, all the women I have known to suffer this have had baby girls. I even managed to guess a womans babys gender the other week..and was correct ;o)
Not that I would like you to quote me on it!