I used to have to stand on one leg with the other knee to my chest, which im pretty sure you cannot do in third trimester. I have had it for 13 years, some times so bad I cant stand on it, the rest of the time, cant sleep on it. Runs all the way from buttocks to foot, on my left leg.
I dont know of anything to help it. --maybe electric therapy--
chiroprator, or physical therapy has them.