I too have sciatica pain. Stretching or doing some squats helps. Sitting usually makes it worse. My lower back will hurt if I sit too long or if I'm on my feet too long. Also been getting leg cramps in my calves, again, stretching and moving usually helps. I have been carrying really low the whole pregnancy and the Dr said that the baby is probably just putting a lot of pressure on those nerves. When it gets rely bad a warm bath helps too.
I carry low so my baby is on my sciatica or however you spell it. Which equals constant lower back, butt and the back of my right thigh pain. I've just been sitting on a heating pad that's on the lowest setting or trying to change positions. I plan on asking my OB the next time I go if creams like Bengay or Hiland's Leg Cramps are okay to use. Also, if you're ok with getting in and out, you might try a warmer bath with epsom salt. Helps to relax muscles. Honestly though, you're probably just in for a bit of a rough ride. That's a really common pregnancy complaint/symptom. :(
I'm just over 21 weeks and I don't have any back or butt pain. With my first two kids, I definitely had the back pain, especially during dishes and other housework. But I don't remember how far along it started. Sorry I can't help more.