Your kidneys are a lot lower, like right on the ridges of your pelvis in the back. Anyways, I don't know where the bra strap pain is coming from. I have it too, but not as bad (33 weeks). It's a good possibility our bellies are messing with the muscles there, but ask you doctor at the next appointment. Maybe there's something they know of that can help.
BTW, if anyone has burning skin pain right under the bra in the front, my doctor says that's from stretching and to put Aquaphor on it. It relieved it pretty well after a day, but I keep reapplying twice a day to keep it from burning.