I get this all the time too, especially when i've been sitting at my desk for a while and have to get up to go somewhere. It usually also results in a slow waddle walk until things loosen up a bit. I think its because when i sit at my desk i sit really forward and legs wide to make room for my tummy which then means the weight of the baby's head is sitting on that reigion (My baby has been head down and partly engaged since 28 weeks or so, though she hasn't moved dwn any further as at 34 weeks).

My midwife suggested sitting like this to make room for the baby and encourage it to engage. I have also been told by the same midwife and again by a different midwife at our classes the worst thing to do is sit in a reclined position and put your feet up as the baby will move into a bad position as it takes the easy way out, which can mean in labour its going to take more time to make up it's mind about how it's going to come out. It's a bit hard to explain but I hope I made some sense at least.