I'm only 32 weeks along and have been feeling really achey for the past week. I had a break from it for maybe 10 weeks (started feeling it a lot around 20 weeks and then it went away with an office chair adjustment). My ob suggested prenatal yoga to strengthen my core muscles, and I'm going to give it a go- but it sounds painful!! She said that it's about that time when you can't do anything for too long of a period, so to keep shifting (sitting, standing, walking, stretching, etc.). Hang in there mama!
I also thought about getting one of those big exercise balls to sit on, hoping that it would help strengthen and stretch me out.