I have been since about 30 weeks as well. Its your hips spreading as (I think this is the name of it) the flexitol (might be flexitrol) in your system starts to get your body ready for birth. The baby is growing super fast right now so your going to have that bearing down pressure from here on out. It gets worse when they drop.
If the pain is more in your groin and lower butt, its probably the baby sitting on your sciatic nerve which starts around this time as well. The joys of late pregnancy right? lol