I had the same. I was told it was just the baby. Do you go for monthly prenatal checkups? (We do in Canada.) You can mention it to your doctor then.
Personally, my advice would be to put your feet up and rest. Drink plenty of fluids (not caffeinated) and eat your fruits, vegetables and whole grains. See if you feel a little better after that. You might just be constipated. :) (Oh the joys of pregnancy!)