If it just happens on the right side it could be sciatic pain. I have had it on and off for many years and it flared up again during my pregnancy and it was exactly as you described it. Mine didn't get any worse as I got further along, but I do still do get pain from it from time to time. If it's both sides or on the left it's probably something else related to the pregnancy, but that would be a good thing cos at least when you are no longer pregnant you wouldn't get it again (unlike sciatica).