pinching and pressure

Has anyone had the sensation that someone is pinching them from the inside? It almost feels like the muscles are being pulled or something. It isn't necessarily painful but it isn't comfortable either. What is that?!

I've also begun to experience a lot of pressure today. It feels like my pubic area is sore and I'm waddling quite a bit more. Neither of my babies was head down at my last appt so I'm hoping that has changed and this is a good sign. But at only 36 1/2 weeks I'm wondering how long I can handle all this pressure!

My two children were little, 5 lbs 14 oz and 7 lbs 1 oz, so having two 6+ lb babies in there is a new experience for me. They're both bigger than my DD was at birth. I'm not sure what is normal. I've also had such great pregnancies so far that this is a whole new ballgame too. I was walking 3-5 miles a day at this point in those pregnancies and feeling great. Not so this time, I'm beat, and walking is not something I like to do today (even if it is just to get a glass of water!)

So, for those that made it this far, what was your experience? What are these sensations? And, assuming that this pressure is now a baby's head in my pelvis, how long can I expect to keep these babies in? My other two children engaged early, very early, and stayed until 39 weeks. But they were small like I said and didn't have another 6+ lb sibling on top of them. haha.