Walking, running, and defying logic...

Over the last two to three weeks, I've been having a lot of curious pains and I'd like to know if others have experienced this. I've already talked to my OB and my MW about them, so I'm not asking for medical advice, but I am asking for an explanation of this strange occurrence.

I can go for a run (it's slow now, think 9 minute/mi.) and I don't have any abdominal cramping or tightening during the run. Immediately following the run, I do a cool down walk. Here's the strange part. Whether I'm walking to cool down from a run, walking through the store, or walking our dog in the evening, I have extreme tightening to the point of significant pain in my lower uterus (stop you in your tracks kind of pain that comes in waves). It isn't a hydration thing. Is it the length of my stride or the way I walk versus how my posture is held when I run?

I've been playing with shortening my walking stride and still going the same speed, and so far the only adjustment I've made that makes a difference is slowing my pace to a grandma walk (not a grandma "mall walk", but a true grandma walk). Does anyone else have similar experiences in this point in the pregnancy? I'm 29w1d and I've had it since mid-27th week.

TIA!