Sounds similar to the pain I am getting. I have had it since I was 6 weeks preggers. I was referred to the Womens Health Physio at the Maternity Ward as I have a problem with my sacrum (sp?) joint in my lower back which causes pain in my left cheek (not the one on my face!!). Basically, I already have a problem with lax joints but during pregnancy you produce the hormone Relaxin which makes your joints loosen up. Unfortunately mine are a little too loose now and it cause pain when I bend in a certain position or try to lift my left leg up (putting trousers on is a bit of a challenge).
I would suggest you ring your midwife up and see about getting referred to a Womens Health Physio. Don't see a normal physio though, you need to see someone that specialises in pregnant women as there are certain treatments you cannot have during pregnancy.
I started off having accupuncture but after 3 sessions they realised it wasn't working so I am now booked in for Hydrotherapy - hopefully that will ease it a little but unfortunately nothing will get rid of it until after the baby is born.
Sorry it's a long reply here but hopefully you can get something sorted that helps ease your pains.
Take care,
karen x