Any tips on being able to distinguish uterine cramps from constipation (or anything else benign)?

I seem to be fine if I'm still. But I got up to use the bathroom early this morning and was immediately in pain. It took a while to find a position that was comfortable and get back to sleep.

Now at work, I was ok sitting at my desk for an hour. I got up to go to the printer, and it was hard to just straighten up.

It's a lasting dull ache (not quick and sharp). It's low, so I can't really tell if it's intestinal or uterine. I'm about 13.5 weeks pregnant.

(TMI...I have had a BM today, but I'm not really convinced I've "emptied" my bowels...)

Secondarily, at what point would you call your OB about pain? I just had the NT scan Friday and everything was fine. I'm scheduled to see my OB in a week.