Ask your doctor to check your bile salts. I had the same problem and they checked that and it came back negative but apparently it is a problem with your liver. Has your skin been yellow at all? The doc said that is a good indication. Mine ended up being that i was allergic to the chlorine at the hospital pregnancy water classes.
Having probs with your bile salts is nothing too serious it just means the bub will come out early.

I hope this helps