I had this with dd1, and it is agony. You have my sympathies. I got to the point where I could only walk for any length of time if bombed out of my head on prescription-strength painkillers.
Firstly, get some pain relief. Most of the powerful painkillers are OK in the middle months of pregnancy, just not at the beginning or at the end (in case you go into labour and the baby is affected).
Secondly, go to a chiropracter who is experienced at dealing with pregnant women. They will assess you and will be able to offer some manipulation that may actually make a difference, unlike physios, who will do a lengthy assessment and then tell you there's nothing they can do. (That was my experience, anyway).
Thirdly, get the best supportive elasticated pregnancy belt you can lay your hands on. It won't make a huge difference but it may help a bit.
Pregnancy is not a great time to start Pilates, but if you can face it, I would strongly suggest you go to a studio and have some one-to-one sessions to try and strengthen your supporting muscles. Go to a specialist studio, not an ordinary gym, and talk to the head honcho. They may even offer the option of someone coming to you. And once you've recovered from the birth, take up Pilates more seriously, as your back will still be very vulnerable, esp. with a baby to lift and carry. I put a disc out at 6 weeks post-partum, and believe me you want to avoid that.
Do feel free to CAT me if you want more info or details.
hth
hth